Methodology and apparatus to terminate abandoned active implantable medical device leads

ABSTRACT

An energy management system facilitates the transfer of high frequency energy coupled into an implanted abandoned lead at a selected RF frequency or frequency band, to an energy dissipating surface. This is accomplished by conductively coupling the implanted abandoned lead to the energy dissipating surface of an abandoned lead cap through an energy diversion circuit including one or more passive electronic network components whose impedance characteristics are at least partially tuned to the implanted abandoned lead&#39;s impedance characteristics.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 12/693,836filed Jan. 26, 2010 which is now U.S. Pat. No. 8,000,801, which claimsbenefit of 61/147,432 filed Jan. 26, 2009, and is a Continuation-in-partof Ser. No. 12/686,137 filed Jan. 12, 2010 which is acontinuation-in-part of Ser. No. 12/489,921 filed Jun. 23, 2009 which isnow U.S. Pat. No. 7,751,903 and claims benefit of 61/149,833 filed Feb.4, 2009 and claims benefit of 61/144,102 filed Jan. 12, 2009.application Ser. No. 12/489,921 now U.S. Pat. No. 7,751,903 is also acontinuation-in-part of Ser. No. 10/123,534 filed Apr. 15, 2002 now U.S.Pat. No. 7,844,319 which claims benefit of 60/283,725 filed Apr. 13,2001.

BACKGROUND OF THE INVENTION

This invention generally relates to the problem of energy induced ontoabandoned implanted leads during medical diagnostic procedures such asmagnetic resonant imaging (MRI). Specifically, the radio frequency (RF)pulsed field of MRI can couple to an implanted lead in such a way thatelectromagnetic forces (EMFs) are induced in the lead. The amount ofenergy that is induced is related to a number of complex factors, but ingeneral, is dependent upon the local electric field that is tangent tolead and the integral of the electric field strength along the lead. Incertain situations, these EMFs can cause currents to flow into distalelectrodes or in the electrode interface with body tissue. It has beendocumented that when this current becomes excessive, that overheating ofsaid lead or its associated electrode or overheating of the associatedinterface with body tissue can occur. There have been cases of damage tosuch body tissue which has resulted in loss of capture of cardiacpacemaking pulses, tissue damage, severe enough to result in braindamage or multiple amputations, and the like. The present inventionrelates generally to methods of redirecting said energy to a novelenergy dissipating abandoned lead proximal end cap rather than the leadbody or a distal tip electrode-to-tissue interface.

There are many reasons why cardiac rhythm device lead wires areabandoned. These include loss of pacing capture or a high impedance atthe distal electrode to tissue interface. Another reason includes leadbreakage or damage to lead insulation. Yet another reason would besimply due to replacement and relocation of the active implantablemedical device (AIMD). Removal of implanted lead wires is not an easyprocess, particularly after they've been implanted for a long period oftime. Reference is made to a paper given at the 28^(th) AnnualScientific Sessions of the Heart Rhythm Society, in Session 113 onFriday, May 11, 2007 by Dr. Bruce L. Wilkoff, M. D. of the ClevelandClinic Foundation and was entitled, ICD LEAD EXTRACTION OF INFECTEDAND/OR REDUNDANT LEADS. The slides from that paper are incorporatedherein by reference and will be referred to again simply as the Wilkoffreference. Referring to various Figures in the paper, one can see theamount of tissue that is adhering to the leads as they are extracted.During extraction procedures, there are various cutting tools and lasertools that are slipped down over the lead that are used to dislodge thelead from surrounding tissue growth. This is a very delicate processbecause it's a tortuous path. For example, if the laser or mechanicalcutting tool were to penetrate while going around a corner of an arterywall, this would result in a life-threatening situation for the patient.Accordingly, lead wires are often simply abandoned and then clipped offor capped and left inside the patient. As mentioned, the above exampleswere for implanted cardiac rhythm device leads. There are also manyreasons why leads are abandoned for neurostimulators and other types ofAIMDs. Spinal cord, deep brain or cochlear leads are often abandonedsimply because the electrodes are so difficult to extract. The presentinvention is applicable to all types of abandoned implanted leads.

It's also been demonstrated in the literature that abandoned lead wirescan be quite dangerous during magnetic resonance imaging (MRI)procedures. That is, the energy that is coupled from the pulsed RF fieldof the magnetic resonance imaging field creates significant energy inthe lead wire system. In most pacemakers and cardioverterdefibrillators, there is an EMI filter that's present at the point oflead wire ingress through the hermetic titanium housing of the device.These prior art feedthrough capacitors (or monolithic chip capacitors)form a fairly low impedance at MRI RF pulse frequencies. The RF pulsedfrequency for a 1.5 Tesla MR scanner is approximately 64 MHz. For a 3Telsa scanner, the RF pulsed frequency is approximately 128 MHz. Thecapacitive reactance of the prior art EMI feedthrough capacitors isgenerally below 2 ohms at these frequencies. Therefore, when thepacemaker or ICD is plugged into the proximal end of the lead wire, muchof the RF energy from MRI is shunted to the titanium can or housing ofthe AIMD. This is why there have been some reports of warming of thepectoral pocket during MR scans. One is referred to a paper given atHeart Rhythm 2007 by Dr. Rod Gimbal. He reported on a number of ICDpatients, including one patient who reported warming of the pectoralpocket area during the MR scan. When the physician placed his hand overthe patient's AIMD in the pocket area, the doctor himself could feel theheat radiating into his own hand. This could have been caused by MRgradient field eddy current heating, however, it is more likely that theheating was caused by transfer of energy through the feedthroughcapacitor to the housing of the cardiac pacemaker.

Accordingly, when a pacemaker or ICD is unplugged and the abandonedlead(s) is capped or cut off, there is no path for the energy to escapeat the proximal end into the surrounding tissues. Instead, what happensis the energy instead dissipates at the still-connected distal tipelectrode or distal ring electrode. Severe overheating has beendocumented in the literature, including burns to cardiac tissue.Therefore, what is needed is an abandoned lead cap that is capable oftransferring energy at the proximal end. For example, for a cardiacpacemaker, this would typically be in the pectoral pocket where thepacemaker was previously removed. In some cases, a new pacemaker withnew leads is implanted in the same pectoral pocket as the abandonedleads. The pectoral pocket, which embodies fat and muscle tissues, isnot nearly as sensitive to thermal injury as compared to myocardialtissue, the spinal cord, or deep brain tissue. For all of these types ofAMIDs, pectoral implants are common. Similar analogies can be made forspinal cord stimulators and other types of neuromodulation systems. Inother words, when one makes the choice, it will be better to slightlyoverheat the pectoral muscle than it would to overheat the distalelectrode tissue interface. It will be obvious that overheating ofmyocardial, nerves or brain tissue can be debilitating or even lifethreatening. It is also a feature of the present invention to dissipatesaid energy over a large enough surface area of a novel abandoned leadcap such as to prevent overheating to the point where temperature risewould result in thermal injury.

MRI is one of medicine's most valuable diagnostic tools. MRI is, ofcourse, extensively used for imaging, but is also used forinterventional medicine (surgery). In addition, MRI is used in real timeto guide ablation catheters, neurostimulator tips, deep brain probes andthe like. An absolute contraindication for pacemaker or neurostimulatorpatients means that these patients are excluded from MRI. This isparticularly true of scans of the thorax and abdominal areas. Because ofMRI's incredible value as a diagnostic tool for imaging organs and otherbody tissues, many physicians simply take the risk and go ahead andperform MRI on a pacemaker patient. The literature indicates a number ofprecautions that physicians should take in this case, including limitingthe power of the MRI RF Pulsed field (Specific Absorption Rate—SARlevel), programming the pacemaker to fixed or asynchronous pacing mode,and then careful reprogramming and evaluation of the pacemaker andpatient after the procedure is complete. There have been reports oflatent problems with cardiac pacemakers or other AIMDs after an MRIprocedure sometimes occurring many days later. Moreover, there are anumber of recent papers that indicate that the SAR level is not entirelypredictive of the heating that would be found in implanted leads ordevices. For example, for magnetic resonance imaging devices operatingat the same magnetic field strength and also at the same SAR level,considerable variations have been found relative to heating of implantedleads. It is speculated that SAR level alone is not a good predictor ofwhether or not an implanted device or its associated lead system willoverheat.

There are three types of electromagnetic fields used in an MRI unit. Thefirst type is the main static magnetic field designated B₀ which is usedto align protons in body tissue. The field strength varies from 0.5 to3.0 Tesla in most of the currently available MRI units in clinical use.Some of the newer MRI system fields can go as high as 4 to 5 Tesla. Atthe recent International Society for Magnetic Resonance in Medicine(ISMRM), which was held on 5-6 Nov. 2005, it was reported that certainresearch systems are going up as high as 11.7 Tesla and will be readysometime in 2010. This is over 100,000 times the magnetic field strengthof the earth. A static magnetic field can induce powerful mechanicalforces and torque on any magnetic materials implanted within thepatient. This would include certain components within the cardiacpacemaker itself and/or lead systems. It is not likely (other thansudden system shut down) that the static MRI magnetic field can inducecurrents into the pacemaker lead system and hence into the pacemakeritself. It is a basic principle of physics that a magnetic field musteither be time-varying as it cuts across the conductor, or the conductoritself must move within a specifically varying magnetic field forcurrents to be induced.

The second type of field produced by magnetic resonance imaging is thepulsed RF field which is generated by the body coil or head coil. Thisis used to change the energy state of the protons and elicit MRI signalsfrom tissue. The RF field is homogeneous in the central region and hastwo main components: (1) the electric field is circularly polarized inthe actual plane; and (2) the H field, sometimes generally referred toas the net magnetic field in matter, is related to the electric field byMaxwell's equations and is relatively uniform. In general, the RF fieldis switched on and off during measurements and usually has a frequencyof 21 MHz to 64 MHz to 128 MHz depending upon the static magnetic fieldstrength. The frequency of the RF pulse for hydrogen scans varies by theLamor equation with the field strength of the main static field where:RF PULSED FREQUENCY in MHz=(42.56) (STATIC FIELD STRENGTH IN TESLA).There are also phosphorous and other types of scanners wherein the Lamorequation would be different. The present invention applies to all suchscanners.

The third type of electromagnetic field is the time-varying magneticgradient fields designated B_(X), B_(Y), B_(Z), which are used forspatial localization. These change their strength along differentorientations and operating frequencies on the order of 1 kHz. Thevectors of the magnetic field gradients in the X, Y and Z directions areproduced by three sets of orthogonally positioned coils and are switchedon only during the measurements. In some cases, the gradient field hasbeen shown to elevate natural heart rhythms (heart beat). This is notcompletely understood, but it is a repeatable phenomenon. The gradientfield is not considered by many researchers to create any other adverseeffects.

It is instructive to note how voltages and electro-magnetic interference(EMI) are induced into an implanted lead system. At very low frequency(VLF), voltages are induced at the input to the cardiac pacemaker ascurrents circulate throughout the patient's body and create voltagedrops. Because of the vector displacement between the pacemaker housingand, for example, the tip electrode, voltage drop across the resistanceof body tissues may be sensed due to Ohms Law and the circulatingcurrent of the RF signal. At higher frequencies, the implanted leadsystems actually act as antennas where voltages (EMFs) are induced alongtheir length. These antennas are not very efficient due to the dampingeffects of body tissue; however, this can often be offset by extremelyhigh power fields (such as MRI pulsed fields) and/or body resonances.

Magnetic field coupling into an implanted lead system is based on loopareas. For example, in an AIMD abandoned lead, there is a loop formed bythe lead as it comes from the abandoned lead proximal tip to its distaltip electrode, for example, located in the right ventricle. The returnpath is through body fluid and tissue generally straight from the tipelectrode in the right ventricle back up to the abandoned lead cap orend. This forms an enclosed area which can be measured from patientX-rays in square centimeters. Per ANSI/AAMI National Standard PC69, theaverage loop area is 200 to 225 square centimeters. This is an averageand is subject to great statistical variation. For example, in a largeadult patient with an abdominal pacemaker implant, the implanted looparea is much larger (around 400 square centimeters).

Relating now to the specific case of MRI, the magnetic gradient fieldswould be induced through enclosed loop areas. However, the pulsed RFfields, which are generated by the body coil, would be primarily inducedinto the lead system by antenna action. Subjected to RF frequencies, thelead itself can exhibit complex transmission line behavior.

At the frequencies of interest in MRI, RF energy can be absorbed andconverted to heat. The power deposited by RF pulses during MRI iscomplex and is dependent upon the power (Specific Absorption Rate (SAR)Level) and duration of the RF pulse, the transmitted frequency, thenumber of RF pulses applied per unit time, and the type of configurationof the RF transmitter coil used. The amount of heating also depends uponthe volume of tissue imaged, the electrical resistivity of tissue andthe configuration of the anatomical region imaged. There are also anumber of other variables that depend on the placement in the human bodyof the AIMD and the length and trajectory of its associated lead(s). Forexample, it will make a difference how much EMF is induced into apacemaker lead system as to whether it is a left or right pectoralimplant. In addition, the routing of the lead and the lead length arealso very critical as to the amount of induced current and heating thatwould occur. Also, distal tip design is very important as it can heat updue to MRI RF induced energy. The cause of heating in an MRI environmentis twofold: (a) RF field coupling to the lead can occur which inducessignificant local heating; and (b) currents induced between the distaltip and tissue during MRI RF pulse transmission sequences can causelocal Ohms Law heating in tissue next to the distal tip electrode of theimplanted lead. The RF field of an MRI scanner can produce enough energyto induce RF voltages in an implanted lead and resulting currentssufficient to damage some of the adjacent myocardial tissue. Tissueablation (destruction resulting in scars) has also been observed. Theeffects of this heating are not readily detectable by monitoring duringthe MRI. Indications that heating has occurred would include an increasein pacing threshold, venous ablation, Larynx or esophageal ablation,myocardial perforation and lead penetration, or even arrhythmias causedby scar tissue. Such long term heating effects of MRI have not been wellstudied yet for all types of AIMD lead geometries. There can also belocalized heating problems associated with various types of electrodesin addition to tip electrodes. This includes ring electrodes or padelectrodes. Ring electrodes are commonly used with a wide variety ofabandoned implanted device leads including cardiac pacemakers, andneurostimulators, and the like. Pad electrodes are very common inneurostimulator applications. For example, spinal cord stimulators ordeep brain stimulators can include a plurality of pad electrodes to makecontact with nerve tissue. A good example of this also occurs in acochlear implant. In a typical cochlear implant there would be sixteenpad electrodes placed up into the cochlea. Several of these padelectrodes make contact with auditory nerves.

Just variations in the pacemaker lead length and implant trajectory cansignificantly affect how much heat is generated. A paper entitled,HEATING AROUND INTRAVASCULAR GUIDEWIRES BY RESONATING RF WAVES byKonings, et al., Journal of Magnetic Resonance Imaging, Issue 12:79-85(2000), does an excellent job of explaining how the RF fields from MRIscanners can couple into implanted leads. The paper includes both atheoretical approach and actual temperature measurements. In aworst-case, they measured temperature rises of up to 74 degrees C. after30 seconds of scanning exposure. The contents of this paper areincorporated herein by reference.

The effect of an MRI system on the abandoned leads of pacemakers, ICDs,neurostimulators and the like, depends on various factors, including thestrength of the static magnetic field, the pulse sequence, the strengthof RF field, the anatomic region being imaged, and many other factors.Further complicating this is the fact that each patient's condition andphysiology is different and each lead implant has a different lengthand/or implant trajectory in body tissues. Most experts still concludethat MRI for the pacemaker patient should not be considered safe.

It is well known that many of the undesirable effects in an abandonedimplanted lead system from MRI and other medical diagnostic proceduresare related to undesirable induced EMFs in the lead system and/or RFcurrents in its distal tip (or ring) electrodes. This can lead tooverheating of body tissue at or adjacent to the distal tip.

Distal tip electrodes can be unipolar, bipolar and the like. It is veryimportant that excessive current not flow at the interface between thelead distal tip electrode and body tissue. In a typical cardiacpacemaker, for example, the distal tip electrode can be passive or of ascrew-in helix type as will be more fully described. In any event, it isvery important that excessive RF current not flow at this junctionbetween the distal tip electrode and for example, myocardial or nervetissue. Excessive current at the distal electrode to tissue interfacecan cause excessive heating to the point where tissue ablation or evenperforation can occur. This can be life threatening for cardiacpatients. For neurostimulator patients, such as deep brain stimulatorpatients, thermal injury can cause permanent disability or even be lifethreatening. Similar issues exist for spinal cord stimulator patients,cochlear implant patients and the like.

A very important and life-threatening problem is to be able to controloverheating of abandoned implanted leads during an MRI procedure. Anovel and very effective approach to this is to first install parallelresonant inductor and capacitor bandstop filters at or near the distalelectrode of implanted leads. For cardiac pacemaker, these are typicallyknown as the tip and ring electrodes. One is referred to U.S. Pat. No.7,363,090; US 2007/0112398 A1; US 2008/0071313 A1; US 2008/0049376 A1;US 2008/0024912 A1; US 2008/0132987 A1; and US 2008/0116997 A1, thecontents of all of which are incorporated herein. Referring now to US2007/0112398 A1, the invention therein relates generally to L-C bandstopfilter assemblies, particularly of the type used in active implantablemedical devices (AIMDs) such as cardiac pacemakers, cardioverterdefibrillators, neurostimulators and the like, which raise the impedanceof internal electronic or related wiring components of the medicaldevice at selected frequencies in order to reduce or eliminate currentsinduced from undesirable electromagnetic interference (EMI) signals.

U.S. Pat. No. 7,363,090 and US 2007/0112398 A1 show resonant L-Cbandstop filters placed at the distal tip and/or at various locationsalong the medical device leads or circuits. These L-C bandstop filtersinhibit or prevent current from circulating at selected frequencies ofthe medical therapeutic device. For example, for an MRI system operatingat 1.5 Tesla, the pulse RF frequency is 64 MHz, as described by theLamour Equation for hydrogen. The L-C bandstop filter can be designed toresonate at or near 64 MHz and thus create a high impedance (ideally anopen circuit) in the lead system at that selected frequency. Forexample, the L-C bandstop filter, when placed at the distal tipelectrode of a pacemaker lead, will significantly reduce RF currentsfrom flowing through the distal tip electrode and into body tissue. TheL-C bandstop filter also reduces EMI from flowing in the leads of apacemaker, for example, thereby providing added EMI protection tosensitive electronic circuits. In general, the problem associated withabandoned leads is minimized when there is a bandstop filter placed ator adjacent to its distal tip electrodes. However, experiments haveshown that even when such a bandstop filter is present, if the AIMD isdisconnected, distal tip heating can still occur. This is generally dueto the fact that prior art EMI filters located in the AIMD shunt some ofthe MRI induced RF energy out of the leads to the generally conductivehousing of the AIMD. In this case, the AIMD housing, such as the housingof a cardiac pacemaker, acts as an important energy dissipating surface(EDS surface). In general, when the housing of the AIMD acts as an EDSsurface, it does not rise in temperature very much due to its very largesurface area and energy dissipating surface. However, when the lead isabandoned, in other words, the AIMD is removed; the proximal end of thelead is now terminated either in body tissue or in an insulatedabandoned lead cap. It no longer is associated with an EDS surface oreven a means to couple or divert energy to an EDS surface. Accordingly,MRI induced RF energy reflects off this open circuit and goes right onback to the distal electrodes where it can bounce back and forth andcause overheating, even when a distal bandstop filter is present. Itwill be appreciated that all of the embodiments described therein areequally applicable to a wide range of other implantable and externalmedical devices, including deep brain stimulators, spinal cordstimulators, drug pumps, probes, catheters and the like.

Electrically engineering a capacitor in parallel with an inductor isknown as a bandstop filter or tank circuit. It is also well known thatwhen a near-ideal L-C bandstop filter is at its resonant frequency, itwill present a very high impedance. Since MRI equipment produces verylarge RF pulsed fields operating at discrete frequencies, this is anideal situation for a specific resonant bandstop filter. Bandstopfilters are more efficient for eliminating one single frequency thanbroadband filters. Because the L-C bandstop filter is targeted at thisone frequency, it can be much smaller and volumetrically efficient.

A major challenge for designing an L-C bandstop filter for human implantis that it must be very small in size, biocompatible, and highlyreliable. Coaxial geometry is preferred. The reason that coaxial ispreferred is that implanted leads are placed at locations in the humanbody primarily by one of two main methods. These include guide wire leadinsertion. For example, in a cardiac pacemaker application, a pectoralpocket is created. Then, the physician makes a small incision betweenthe ribs and accesses the subclavian vein. The pacemaker leads arestylus guided/routed down through this venous system through thesuperior vena cava, through the right atrium, through the tricuspidvalve and into, for example, the right ventricle. Another primary methodof implanting leads (particularly for neurostimulators) in the humanbody is by tunneling. In tunneling, a surgeon uses special tools totunnel under the skin and through the muscle, for example, up throughthe neck to access the Vagus nerve or the deep brain. In bothtechniques, it is very important that the leads and their associatedelectrodes at the distal tips be very small. US2007/0112398 A1 solvesthese issues by using very novel miniature coaxial or rectilinearcapacitors that have been adapted with an inductance element to providea parallel L-C bandstop filter circuit.

The value of the capacitance and the associated parallel inductor can beadjusted to achieve a specific resonant frequency (SRF). The bandstopfilters described in US 2007/0112398 A1 can be adapted to a number oflocations within the overall implantable medical device system. That is,the L-C bandstop filter can be incorporated at or near any part of themedical device implanted lead system or at or adjacent to the distal tipelectrodes. In addition, the L-C bandstop filter can be placed anywherealong the implanted lead system.

The L-C bandstop filters are also designed to work in concert with anEMI filter which is typically used at the point of lead ingress andegress of the active implantable medical device. For example, see U.S.Pat. No. 5,333,095; U.S. Pat. No. 5,905,627; U.S. Pat. No. 5,896,627;and U.S. Pat. No. 6,765,779, the contents of all being incorporatedherein by reference. All four of these documents describe low pass EMIfilter circuits. Accordingly, the L-C bandstop filters, as described inU.S. Pat. No. 7,393,090, entitled BANDSTOP FILTER EMPLOYING A CAPACITORAND INDUCTOR TANK CIRCUIT TO ENHANCE MRI COMPATIBILITY OF ACTIVEIMPLANTABLE MEDICAL DEVICES, are designed to be used in concert withthese prior art low pass filters. However, when an AIMD lead isabandoned, the filter capacitors, as previously described, are no longerconnected to the lead. Bandstop filters, in accordance with U.S. PatentApplication Publication No. US2007/0112398 A1, work particularly wellwhen the proximal lead is connected to a pacemaker or ICD that has afeedthrough capacitor EMI filter. When the AIMD is disconnected, thereis no place for the energy to go at the proximal end. Accordingly, theenergy is reflected back to the distal tip. Recent testing by theinventors demonstrates that in certain lead configurations, even with abandstop filter present, excessive heating at the distal tip electrodecan still occur.

When one performs MRI testing on an abandoned lead system, one firstestablishes a controlled measurement. That is, with worst-case MRIequipment settings and a worst-case location within the MRI bore, and aworst-case lead configuration, one can measure heating using fiber opticprobes at the distal electrodes. Temperature rises of 30 to over 60degrees C. have been documented. When one takes the same control leadand places miniature bandstop filters in accordance with U.S. Pat. No.7,363,090 or US 2007/0112398 A1, one finds that substantially less MRIinduced energy is directed to distal electrodes greatly reducing theirtendency to overheat. In fact, in many measurements made by theinventors, temperature rises of over 30 degrees C. have been reduced toless than 3 degrees C. However, a secondary problem has been discovered.That is, the implanted lead acts very much as like a transmission line.When one creates a very high impedance at the distal electrode to tissueinterface by installation of a resonant bandstop filter as described inU.S. Pat. No. 7,038,900 and as further described in US 2007/0112398 A1,there is created an almost open circuit which is the equivalent of anunterminated transmission line. This causes a reflection of MRI inducedRF energy back towards the proximal end where the AIMD (for example, apacemaker) would have been connected. However, for an abandoned lead,this creates an open circuit at the proximal end with no place for theenergy to escape. Therefore, this energy can be reflected back and forthresulting in temperature rises along the lead and more particularly atthe distal electrode to tissue interface. In order to completely controlthe induced energy in an abandoned implanted lead system, one must takea system approach. In particular, a methodology is needed whereby energycan be dissipated from the lead system at the proximal end in a way thatdoes not cause overheating either at the distal electrode interface orat the proximal end cap. Maximizing energy transfer from an implantedlead is more thoroughly described in U.S. patent Ser. No. 12/686,137,the contents of which are incorporated herein by reference.

Accordingly, there is a need for controlling the induced energy in animplanted abandoned lead system. Moreover, there is a need for noveltuned RF diverting circuits coupled to one or more energy or heatdissipation surfaces associated with an abandoned lead cap, which arepreferably frequency selective and are constructed of passivecomponents. Such circuits are needed to prevent MRI induced energy fromreaching the distal tip electrode or its interface with body tissue. Byredirecting said energy to an energy dissipation surface distant fromthe distal electrodes, this minimizes or eliminates hazards associatedwith overheating of said lead and/or its distal electrodes duringdiagnostic procedures, such as MRI. For maximum RF energy transfer outof the lead, frequency selective diverter circuits are needed whichdecouple and transfer energy which is induced onto implanted leads fromthe MRI pulsed RF field to an energy dissipating surface associated withan abandoned lead cap. The present invention fulfills these needs andprovides other related advantages.

SUMMARY OF THE INVENTION

The present invention resides in terminating abandoned AIMD leads whichembodies a novel abandoned lead cap associated with an energydissipating surface. In general, the abandoned lead cap includes anelectrically conductive housing/electrode which works in combinationwith frequency selective circuits so that the housing of the abandonedlead cap works as an energy dissipating surface. The energy dissipatingsurface may be disposed within the blood flow of a patient or comprise aplurality of spaced-apart energy dissipating surfaces. The energydissipating surface may also include one or more slots for reducing eddycurrent heating therein.

In an alternative embodiment, the impedance of the abandoned lead capcan be balanced to the implanted lead impedance such that maximum energyis dissipated within the abandoned lead cap itself. In this case,thermal energy can be dissipated inside the abandoned lead cap, whichhas a controlled thermal mass and a controlled rate of temperature rise.

The system for terminating an abandoned implanted lead to minimizeheating in a high power electromagnetic field environment in accordancewith the present invention, comprises: (1) an implanted abandoned leadhaving a proximal end and a distal end, and impedance characteristics ata selected RF frequency or RF frequency band; (2) an abandoned lead caphaving an energy dissipating surface (EDS surface) which is associatedwith the proximal end of the implanted abandoned lead; and (3) an energydiversion circuit conductively coupling the implanted abandoned lead tothe energy dissipating surface to facilitate transfer to the energydissipating surface of high frequency energy induced on the implantedabandoned lead at the selected RF frequency or frequency band. Thepresent invention includes methods of attachment to an abandoned leadthat has been cut off as well as one which has a proximal connector thathas been abandoned. Methods of conductively coupling the abandoned leadto its associated abandoned lead cap EDS surface include directconnections (short to EDS) or connections through frequency selectiveelectronic component networks (frequency selective diverters).

The novel abandoned lead cap of the present invention works best whenbandstop filters are installed at or near the distal electrode of animplanted lead, wherein the RF energy induced by the MRI pulse field isattenuated from flowing into body tissues and thereby being dissipated.However, when bandstop filters are used, that energy still resides inthe lead system. In other words, by preventing this induced energy fromflowing to sensitive tissues at distal electrode interfaces, a greatdeal has been accomplished; however, it is still important to carefullydissipate the remaining energy that's trapped in the lead system. Forabandoned leads, the most efficient way to do this is to use themetallic housing of the novel abandoned lead cap of the presentinvention.

One type of frequency selective network is a feedthrough capacitor.However, to provide optimal decoupling, one has to refer to the maximumpower transfer theorem. When one has an ideal source, consisting of avoltage source and a series impedance, this is known as a TheveninEquivalent Circuit. It is well known in electrical engineering that totransfer maximum power to a load that the load impedance must be equalto the source impedance. If the source impedance is completelyresistive, for example, 50 ohms, then to transfer maximum power, theload impedance would have to be 50 ohms. When the source impedance isreactive, then to transfer maximum power to another location, the loadimpedance should have the opposite sign of reactance and the sameimpedance and resistance. Referring to a typical implanted lead system,the implanted leads typically appear inductive. Accordingly, having acapacitive EDS diverter circuit associated with the abandoned lead cap,one has at least some cancellation of these imaginary impedance factors.In electrical engineering, the inductance of the lead would be denotedby +jωL. The impedance of the capacitor, on the other hand, is a −j/ωCterm. In the present invention, it's important to know the approximateinductance property of the implanted abandoned lead system, so that anoptimal value of capacitance of the novel abandoned lead cap can beselected such that the +J component is nearly or completely canceled bythe appropriate −J component of the capacitor. Again, refer to thecontents of U.S. patent Ser. No. 12/686,137, the contents of which areincorporated herein by reference, for a more complete description ofimpedance cancellation and Thevenin's maximum power transfer theorem inthis application.

Another way to provide an RF short circuit to the metallic housing ofthe novel abandoned lead cap is to use what is known in the industry anL-C series trap filter. When an inductor and a capacitor appear inseries, it will always be a single frequency at which the inductivereactance is equal and opposite to the capacitive reactance. At thispoint, the L-C trap filter is said to be in resonance. For an ideal trapfilter (one containing zero resistance), at resonance, it would presenta short circuit. U.S. Pat. No. 6,424,234 describes L-C trap filters(also known as notch filters). The '234 patent describes notch filtersfor a completely different purpose and application. FIG. 10 of U.S. Pat.No. 6,424,234 shows notch filter attenuation in the kilohertz frequencyrange. The reason for this was to provide some degree of attenuationagainst low frequency emitters, such as 58 kHz electronic articlesurveillance (store security) gates. These gates detect tags oncommercial items (such as clothing) as an anti-theft detection system.However, in the present invention, L-C trap filters can be optimallytuned to dissipate the RF pulsed energy in an abandoned lead inducedfrom the RF field of an MRI system. For example, for a 1.5 Tesla system,the L-C trap filter would be tuned at the Lamour frequency of 64 MHz.

The present invention additionally resides in an overall energymanagement system capable of controlling the energy induced in abandonedimplanted leads from the RF pulsed field of MRI scanners. Moreparticularly, the present invention resides in a tuned energy balancedsystem for minimizing heating of an abandoned implanted lead in a highpower electromagnetic field environment. The tuned energy balancedsystem of the present invention comprises an abandoned implanted leadhaving impedance characteristics at a selected RF frequency or frequencyband, an energy dissipating surface associated with the abandoned leadcap, and an energy diversion circuit conductively coupling the implantedlead to the energy dissipating surface of the abandoned lead cap. Theenergy diversion circuit comprises one or more passive electronicnetwork components whose impedance characteristics can be partiallytuned to the implanted lead's impedance characteristics, to facilitatetransfer to the energy dissipating surface of high frequency energyinduced on the implanted lead at the selected RF frequency or frequencyband.

The high frequency energy may comprise an MRI frequency or a range ofMRI frequencies selected from the group of frequencies associated withan MRI scanner. In a preferred embodiment, the energy diversion circuithas a reactance that is vectorially opposite to the characteristicreactance of the implanted lead. Moreover, the energy diversion circuithas a capacitive reactance generally equal and opposite to thecharacteristic inductive reactance of the implanted lead. Preferably,the capacitive reactance and the inductive reactance each have aresistor component.

The energy diversion circuit may comprise a low pass filter such as acapacitor, an L-C trap, an “n” element filter or an L-C bandstop filter.Moreover, the energy diversion circuit may comprise one or more seriesresonant LC trap filters.

An impeding circuit may be associated with the energy diversion circuitfor raising the high-frequency impedance of the abandoned lead cap atselected frequency. The impeding circuit may comprise an inductor and/ora bandstop filter.

The energy dissipating surface may comprise convolutions, fins or aroughened surface for increasing the surface area thereof. The roughenedsurface may be formed through plasma or chemical etching, porous orfractal coatings or surfaces, whiskers, morphologically designedcolumbar structures, vapor, electron beam or sputter deposition of ahigh surface area energy conductive material, or carbon nanotubes.

The energy dissipating surface of the abandoned lead cap may comprise amaterial capable of being visualized during magnetic resonance scan.Further, the energy dissipating surface may include a biomimeticcoating.

The energy diversion circuit of the abandoned lead cap may include atleast one non-linear circuit element such as a transient voltagesuppressor, a diode or a pin diode.

The energy diversion circuit of the abandoned lead cap may comprise ahigh pass filter which prevents low frequency gradient field-inducedenergy in the implanted lead or the leadwire from passing through thediversion circuit to the energy dissipating surface. A high pass filtermay comprise a capacitor, a resistor in series with the capacitor, or anL-C trap filter.

The abandoned lead cap housing may include a set screw for locking theproximal end of the implanted abandoned lead within the conductiveblock. The set screw may include a spike, tip or piercing rug tofacilitate conductive coupling of the proximal end of the implantedabandoned lead with the conductive block. Alternatively, the abandonedlead cap housing may include a threaded locking system for locking theproximal end of the implanted abandoned lead within the conductiveblock.

The abandoned lead cap of the present invention may also include an RFIDtag associated therewith.

Other features and advantages of the present invention will becomeapparent from the following more detailed description, taken inconjunction with the accompanying drawings which illustrate, by way ofexample, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate the invention. In such drawings:

FIG. 1 is a wire-formed diagram of a generic human body showing a numberof exemplary implanted medical devices;

FIG. 2 is a diagrammatic view of a typical probe or catheter;

FIG. 3 is an electrical diagrammatic view of the interior of the proberor catheter of FIG. 2;

FIG. 4 is an electrical diagrammatic view of the structure shown in

FIG. 3, with a general impedance element connected between leads;

FIG. 5 is an electrical diagrammatic view similar to FIG. 4,illustrating a capacitor representing a frequency dependent reactiveelement between the leads;

FIG. 6 is a view similar to FIG. 5, wherein the general reactanceelement has been replaced by a capacitor in series with an inductor;

FIG. 7 is a view similar to FIGS. 4-6, showing the addition of seriesfrequency selective reactances;

FIG. 8 is similar to FIG. 3, showing a low frequency model of thecatheter and associated leads described in FIG. 2;

FIG. 9 is a view similar to FIGS. 3-8, illustrating how the distal ringsare electrically isolated at a high frequency;

FIG. 10 is a view similar to FIGS. 3-9, showing the addition of seriesinductor components added to the frequency selective elements 112;

FIG. 11 is similar to FIGS. 3-10, illustrating frequency selectiveelements which incorporate parallel resonant inductor and capacitorbandstop filters;

FIG. 12 is a perspective and somewhat schematic view of a prior artactive implantable medical device (AIMD) including a pair of leadsdirected to the heart of a patient;

FIG. 13 is a perspective view of a dual chamber IS-1 header block;

FIG. 14 is a perspective view of a four chamber header block;

FIG. 15 is a perspective view of an inline quadpolar IS-4 lead;

FIG. 16 is a perspective view of a typical lead system for animplantable cardioverter defibrillator;

FIG. 17 is a schematic illustration of a bipolar lead system with adistal tip and ring typically as used with a cardiac pacemaker;

FIG. 18 is a schematic illustration of a prior art single chamberbipolar cardiac pacemaker lead showing the distal tip and the distalring electrodes;

FIG. 19 is an enlarged, fragmented schematic view taken generally alongthe line 19-19 of FIG. 18, illustrating placement of bandstop filtersadjacent to the distal tip and ring electrodes;

FIG. 20 is a line drawing of a human heart with cardiac pacemaker dualchamber bipolar leads shown in the right ventricle and the right atrium;

FIG. 21 is a line drawing of a exemplary lead systems with various typesof electrode tips;

FIG. 22 is a schematic circuit diagram illustrating an exemplary energydiverter with non-linear circuit elements such as a pair of diodes in aparallel therewith;

FIG. 23 is a view similar to FIG. 22, except showing that the divertercould also be an impeder element;

FIG. 24 is a schematic illustration of a unipolar lead system attachedto an abandoned lead cap of the present invention;

FIG. 25 is an illustration similar to FIG. 24, except that the diverterelement has been combined with an impeder element;

FIG. 26 is an illustration similar to FIG. 24, wherein the diverterelement is shown as a capacitor;

FIG. 27 is an illustration similar to FIG. 26, except that a resistorelement has been added in series with the capacitor;

FIG. 28 is an illustration similar to FIG. 24, except that the diverterelement is replaced by shorting the lead to the housing of the AIMD;

FIG. 29 is an illustration similar to FIG. 24, wherein the diverterelement is shown as a resistor element;

FIG. 30 is an illustration similar to FIG. 24, wherein the diverterelement is shown as an inductor in series with a capacitor;

FIG. 31 is an illustration similar to FIG. 30, wherein a seriesresistance is added to the L-C trap filter;

FIG. 32 is an illustration similar to FIG. 25, wherein the impederelement is shown as a bandstop filter;

FIG. 33 is an illustration similar to FIG. 32, except that the impederelement is a simple inductor;

FIG. 34 is an illustration similar to FIGS. 24-33, showing variousdiverter and impeder elements connected to an energy dissipatingsurface;

FIG. 35 is an illustration of an X-ray tracing of an implanted cardiacpacemaker in a patient;

FIG. 36 is an illustration similar to FIG. 26, where the diverterelement features a specific capacitive element;

FIG. 37 is a high frequency model of the circuit shown in FIG. 36;

FIG. 38 is a low frequency model of the circuit shown in FIG. 36;

FIG. 39 illustrates a typical cardiac IS-1 proximal end connector;

FIG. 40 is a cross-sectional side view of an abandoned lead cap designedto receive the proximal end connector of FIG. 39;

FIG. 41 is a view similar to FIG. 40, except that the IS-1 proximal endconnector of FIG. 39 has been removed;

FIG. 42 is an isometric view of the abandoned lead cap of FIGS. 40 and41;

FIG. 43 is an isometric view of a lead cap similar to that of FIG. 42,except that it has two proximal connector-receiving ports;

FIG. 44 is an isometric view of a lead cap similar to that of FIG. 42,except that it has four proximal connector-receiving ports;

FIG. 45 is a schematic diagram of the unipolar abandoned lead cap ofFIGS. 40-42;

FIG. 46 is a view similar to FIG. 41, showing both an internal metalplate and an external energy dissipating surface plate;

FIG. 47 is a view similar to FIG. 46, showing a capacitor connectedbetween the internal metal plate and the energy dissipating surface;

FIG. 48 is an isometric view of the abandoned lead cap of FIG. 47,except that two capacitors are used;

FIG. 49 is a view similar to FIG. 46, except that the tip and ringelectrode are both shorted to the metal plate and an L-C resonantcircuit is connected between the metal plate and the energy dissipatingsurface;

FIG. 50 is a view similar to FIG. 49, illustrating that the circuitelements can be any of those shown in FIGS. 2-11;

FIG. 51 is a view similar to FIG. 40, except that the metal plate isremoved and the energy dissipating surface is on the outside of theinsulating housing;

FIG. 52 is a view similar to FIG. 41, except that the metal plate isdivided into two electrically isolated plates;

FIG. 53 is a view similar to FIG. 47, except that the capacitor has beenplaced in a hermetic package;

FIG. 54 is a cross-sectional view of another exemplary embodiment of anabandoned lead cap;

FIG. 55 is an isometric view similar to that of FIG. 43, illustrating astreamlined abandoned lead cap configuration;

FIG. 56 is an isometric view similar to that of FIG. 55 from the bottomperspective;

FIG. 57 is an isometric view of an exemplary three port abandoned leadcap;

FIG. 58 is a view similar to FIG. 57, showing slots in the energydissipating surface;

FIG. 59 is an schematic diagram of the structure shown in FIG. 52;

FIG. 60 is a side view of an in-line quadpolar IS-4 connectorincorporating both high and low voltage connections;

FIG. 61 is a side view of an in-line quadpolar IS-4 connectorincorporating only low voltage connections;

FIG. 62 is a cross-sectional side view of a lead cap designed toterminate the abandoned IS-4 lead connectors of FIGS. 60 and 61;

FIG. 63 is an isometric view of the abandoned lead cap of FIG. 62;

FIG. 64 is an electrical schematic of the abandoned lead cap of FIGS. 62and 63;

FIG. 65 is an isometric view of the abandoned lead cap of FIG. 63,showing diverter and impeder elements incorporated therein;

FIG. 66 is an electrical schematic of the abandoned lead cap of FIG. 65;

FIG. 67 illustrates a paddle-shaped abandoned lead cap of the presentinvention;

FIG. 68 is a sectional view taken along line 68-68 of FIG. 67;

FIG. 69 is a perspective view of the proximal end of an IS-1 connector;

FIG. 70 an enlarged, partially fragmented perspective andcross-sectional end view taken along line 70-70 of FIG. 69;

FIG. 71 is a perspective view of an exemplary locking insert withspikes;

FIG. 72 is a cross-sectional view of an exemplary abandoned lead capshowing how the locking insert is utilized to hold a cut-off proximallead in place and simultaneously make proper electrical connection withthe energy dissipating surface;

FIG. 73 is a view similar to FIG. 72, showing another method forterminating abandoned leads using a sharp point set screw;

FIG. 74 is a view similar to FIGS. 72 and 73, showing another method forterminating abandoned leads using a sharp point set screw with asecondary piercing ring;

FIG. 75 is an enlarged cross-sectional side view of an energydissipating surface with a convoluted surface;

FIG. 76 is an enlarged cross-sectional side view of an energydissipating surface with a roughened surface; and

FIG. 77 is view similar to FIG. 76, except that instead of a roughenedsurface, carbon nanotubes or fractal coatings have been added to theenergy dissipating surface.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in the drawings for purposes of illustration, the presentinvention relates to a system for terminating abandoned implanted leadsto minimize heating in high power electromagnetic field environments. Ina broad sense, the present invention comprises an implanted abandonedlead having impedance characteristics at a selected RF frequency orfrequency band, an energy dissipating surface associated with theabandoned lead, and an energy diversion circuit conductively couplingthe abandoned lead to the energy dissipating surface. The energydiversion circuit may comprise one or more passive electronic networkcomponents whose impedance characteristics are at least partially tunedto the abandoned lead's impedance characteristics, to facilitatetransfer to the energy dissipating surface of high frequency energyinduced on the abandoned lead at the selected RF frequency or frequencyband. Certain implanted leads may have a characteristic impedance thatincludes capacitive reactance. In this case the novel energy diversioncircuit of the present invention would include inductive elements inorder to cancel or partially cancel the capacitive reactance of theimplanted lead.

The invention further resides in a combination of one or more bandstopfilters placed at or near the distal electrode-to-tissue interface of animplanted abandoned lead, and a frequency selective diverter ordiversion circuit which decouples energy induced on the implantedabandoned lead at a frequency or frequency band of an interest to anenergy dissipating surface associated with the abandoned lead cap.

There are several reasons why implanted leads are abandoned, whichinclude difficulty in removal, newer technologies and the like. However,a major reason that leads are often abandoned is due to insulationbreakage, insulation resistance problems, or lead fractures. Forexample, when a cardiac pacemaker lead has an insulation defect or alead fracture, it is often difficult or impossible to deliver therapy tothe correct area in myocardial tissue. Stimulation of the pectoralmuscle or phrenic nerve stimulation may also result. Such defectiveleads are often abandoned and replaced with a new lead in parallel.Exposure of the abandoned lead to high RF electromagnetic fieldenvironments, such as during MRI scans, cannot only cause overheating ofdistal electrodes, but also may result in excessive RF current flows inthe area of defective insulation or a lead fracture. The abandoned leadcap is very useful in this regard in that by pulling energy out of theimplanted lead and redirecting it to the EDS surface of the abandonedlead cap, one provides a high degree of protection. Accordingly, thepresent lead cap of the present invention not only protects a distalelectrode to tissue interface, but also protects against other areasalong the lead where RF leakage could occur.

In the case where bandstop filters are installed at or near the distalelectrode of an implanted abandoned lead, the RF energy induced by theMRI pulse field is inhibited from flowing into body tissues and therebybeing dissipated. However, even when distal electrode bandstop filtersare used, that energy still resides in the lead system. In other words,by preventing this induced energy from flowing to sensitive tissues atdistal electrode interfaces, a great deal has been accomplished;however, it is still important to carefully dissipate the remainingenergy that's trapped in the lead system.

In order to provide optimal decoupling of RF energy from an implantedlead to the energy dissipating surface of an abandoned lead cap, oneshould consider Thevenin's maximum power transfer theorem. When one hasan ideal source, consisting of a voltage source and a series impedance,this is known as a Thevenin Equivalent Source Circuit. It is well knownin electrical engineering that to transfer maximum power to a load thatthe load impedance must be equal to the source impedance. If the sourceimpedance is completely resistive, for example, 50 ohms, then totransfer maximum power, the load impedance would have to be 50 ohms.When the source impedance is reactive, then to transfer maximum power toanother location, the load impedance should have the opposite sign ofreactance and the same impedance and resistance. Referring to a typicalimplanted lead system, the implanted leads typically appear inductive.Accordingly, having a capacitive energy diversion circuit within theabandoned lead cap to couple energy from the lead to the EDS surface,one has at least some cancellation of these imaginary impedance factors.In electrical engineering, the inductance of the lead would be denotedby +jωL. The impedance of the capacitor, on the other hand, is a −j/ωCterm. In the present invention, it's important to know the inductanceproperty of the implanted lead system, so that an optimal value ofcapacitance between the EDS surface and ground can be selected such thatthe +J component is nearly or completely canceled by the appropriate −Jcomponent of the capacitor.

For maximal MRI energy dissipation from the lead system, one would wantthe capacitive reactance value to be equal and opposite in value to theinductive reactance of the lead system.

One does not have to exactly match the impedances of an implantedabandoned lead system to the diverter circuits of the present invention.Implanted leads usually tend to be inductive, although in certain casesthey can even be capacitive. What is important is that the divertercircuit has a reactance which is vectorially opposite to thecharacteristic reactance of the implanted lead. In other words, if theimplanted lead is inductive, it will have a +jωL inductive reactance inohms. One would balance this with a −j/ωC capacitive reactance in thediverter circuit. In an ideal case, the reactance of the divertercircuit would be generally equal and opposite to the characteristicreactance of the implanted lead. In an absolutely ideal situation, theimplanted lead would have a characteristic inductive reactance and thediverter circuit would have an equal but opposite vector quantitycapacitive reactance which would cancel. In order to obtain optimalenergy transfer to an EDS surface in this case, it would further enhanceenergy transfer if the diverter circuit also had a resistive value thatis equal to the characteristic resistance of the implanted lead.Fortunately, particularly when used in combination with a bandstopfilter, it is not essential that the impedance or reactance of thediversion circuit be completely equal and opposite to the impedance orreactance of the implanted lead system.

The present invention is ideal for claiming MRI compatibility for arange of abandoned implanted leads. Using a cardiac pacemaker as anexample, one may either through measurement or modeling characterize theimpedance of leads of various lengths, such as 35 to 55 centimeters, andalso analyze their characteristic impedance over various implantanatomical geometries. One could then determine an average impedance orreactance of this range of leads in order to design an averaged oroptimized diverter circuit. Unlike for bandstop filters, the divertercircuit will generally work over a broad range of circuits, not just asingle frequency. Accordingly, by using a properly tuned divertercircuit coupled to an energy dissipation surface of the abandoned leadcap of present invention, one would be able to assure that a range oflead lengths, lead types and implant geometries will all be safe in ahigh electric magnetic field environment such as MRI.

In a first order approximation, the energy diversion circuit of thepresent invention can simply be a short circuit or a resistor which isattached to the characteristic resistance of the average of theimplanted leads for which one claims compliance. For example, if theimplanted leads generally have a resistance value of around 80 ohms,then one could achieve a very high degree of tuned energy balance byhaving an 80 ohm resistor coupled between the lead and the energydissipating surface. This would not cancel the reactance of theabandoned lead system but would still go a long way to remove energyfrom the leads and transfer it to the EDS surface of the abandoned leadcap.

The present invention also includes frequency selective diversion(decoupling) circuits which transfer RF energy which is induced ontoimplanted abandoned leads from a high power electromagnetic fieldenvironment such as an MRI RF field to an energy dissipating surface(EDS surface).

The present invention is primarily directed to the MRI pulsed RF fieldalthough it also has applicability to the gradient field as well.Because of the presence of the powerful static field, non-ferromagneticcomponents are used throughout the present invention. The use offerromagnetic components is contraindicative because they have atendency to saturate or change properties in the presence of the mainstatic field.

FIG. 1 illustrates various types of active implantable medical devicesreferred to generally by the reference numeral 100 that are currently inuse. FIG. 1 is a wire formed diagram of a generic human body showing anumber of exemplary implanted medical devices. 100A is a family ofimplantable hearing devices which can include the group of cochlearimplants, piezoelectric sound bridge transducers and the like. 100Bincludes an entire variety of neurostimulators and brain stimulators.Neurostimulators are used to stimulate the Vagus nerve, for example, totreat epilepsy, obesity and depression. Brain stimulators are similar toa pacemaker-like device and include electrodes implanted deep into thebrain for sensing the onset of the seizure and also providing electricalstimulation to brain tissue to prevent the seizure from actuallyhappening. 100C shows a cardiac pacemaker which is well-known in theart. 100D includes the family of left ventricular assist devices(LVAD's), and artificial hearts, including the recently introducedartificial heart known as the Abiocor. 100E includes an entire family ofdrug pumps which can be used for dispensing of insulin, chemotherapydrugs, pain medications and the like. Insulin pumps are evolving frompassive devices to ones that have sensors and closed loop systems. Thatis, real time monitoring of blood sugar levels will occur. These devicestend to be more sensitive to EMI than passive pumps that have no sensecircuitry or externally implanted leadwires. 100F includes a variety ofimplantable bone growth stimulators for rapid healing of fractures. 100Gincludes urinary incontinence devices. 100H includes the family of painrelief spinal cord stimulators and anti-tremor stimulators. 100H alsoincludes an entire family of other types of neurostimulators used toblock pain. 100I includes a family of implantable cardioverterdefibrillator (ICD) devices and also includes the family of congestiveheart failure devices (CHF). This is also known in the art as cardioresynchronization therapy devices, otherwise known as CRT devices.

Referring to U.S. 2003/0050557, Paragraphs 79 through 82, the contentsof which are incorporated herein, metallic structures, particularlyleads, are described that when placed in MRI scanners, can pick up highelectrical fields which results in local tissue heating. This heatingtends to be most concentrated at the ends of the electrical structure(either at the proximal or distal lead ends). This safety issue can beaddressed using the disclosed systems and methods of the presentinvention. A significant concern is that the distal electrodes, whichare in contact with body tissue, can cause local tissue burns.

As used herein, the lead means an implanted lead, including itselectrodes that are in contact with body tissue. In general, for anAIMD, the term lead means the lead that is outside of the abandoned leadcap housing and is implanted or directed into body tissues. The termleadwire as used herein, refers to the wiring that is generally insideof the abandoned lead cap generally to its EDS surface, circuit board,substrates or internal circuitry. FIGS. 1A through 1G in U.S.2003/0050557 have been redrawn herein as FIGS. 2 through 11 and aredescribed as follows in light of the present invention.

FIG. 2 is a diagrammatic view of a typical prior art device 102 such asa probe, catheter or AIMD lead distal electrode. There are two leads 104and 106 which thread through the center of the illustrative probe,catheter or AIMD lead 102 and terminate respectively in a correspondingpair of distal conductive electrode rings 108 and 110. Leads 104 and 106are electrically insulated from each other and also electricallyinsulated from any metallic structures located within the catheter orlead body. The overall catheter or implanted lead body is generallyflexible and is made of biocompatible materials, which also havespecific thermal properties.

FIG. 3 shows the interior taken from FIG. 2 showing leads 104 and 106which are routed to the two distal electrodes 108 and 110 as previouslydescribed in FIG. 2.

FIG. 4 shows the electrical circuit of FIG. 3 with a general frequencyselective reactive diverting element 112 connected between leads 104 and106. In the present invention, the diverting element 112 can consist ofa number of frequency selective elements as will be further described.In general, the first conductive lead 104 is electrically coupled to thefirst electrode 108, the second conductive lead 106 is electricallycoupled to the second electrode 110, and the frequency dependentreactive diverting element 112 electrically couples the first and secondleads 104 and 106 such that high frequency energy is conducted betweenthe first lead 104 and the second lead 106.

Referring once again to FIG. 4, the frequency selective reactivediverting element 112 tends to be electrically invisible (i.e., a veryhigh impedance) at selected frequencies. The reactive element isdesirably selective such that it would not attenuate, for example, lowfrequency biological signals or RF ablation pulses. However, for highfrequency MRI RF pulsed frequencies (such as 64 MHz), this frequencyreactive diverting element 112 would look more like a short circuit.This would have the effect of sending the energy induced into the leads104 and 106 by the MRI RF field back into the catheter body energydissipating surface into which the leads are embedded. In other words,there are desirably both RF energy and thermal conductivity to the probeor catheter body or sheath or shield which becomes an energy dissipatingsurface all along the lengths of leads 104 and 106 such that MRI inducedenergy that is present in these leads is diverted and converted to heatinto the interior and along the catheter body itself. This prevents theheat build up at the extremely sensitive locations right at the ringelectrodes 108 and 110 which are in intimate and direct contact withbody tissue. In addition, the amount of temperature rise is very small(just a few degrees) because of the energy being dissipated over such arelatively high surface area. As previously mentioned, the highfrequency RF pulsed energy from an MRI system can couple to implantedleads. This creates electromagnetic forces (EMFs) which can result incurrent flowing through the interface between electrodes that are incontact with body tissue. If this current reaches sufficient amplitude,body tissue could be damaged by excessive RF current flow or heatbuild-up. In certain situations, this can be life threatening for thepatient.

FIG. 5 shows a capacitor 114 which represents one form of the frequencyselective diverting reactive element 112. In this case, the reactiveelement 112 comprises a simple capacitor 114 connected between the firstconductor or lead 104 and the second conductor or lead 106 and will havea variable impedance vs. frequency. The following formula is well knownin the art: X_(C)=1/(2πfc). Referring to the foregoing equation, one cansee that since frequency (f) is in the denominator, as the frequencyincreases, the capacitive reactance in ohms decreases. With a largenumber in the denominator, such as the RF pulsed frequency of a 1.5Tesla MRI system, which is 64 MHz, the capacitive reactance drops to avery low number (essentially a short circuit). By shorting the leadstogether at this one frequency, this diverts and prevents the RF energyfrom reaching the distal ring electrodes 108 and 110 and beingundesirably dissipated as heat into body tissue. Referring once again toFIG. 4, one can see that the frequency selective diverting element 112thereby diverts the high frequency RF energy back into the leads 104 and106. By spreading this energy along the length of leads 104 and 106, itis converted to heat, which is dissipated into the main body of theprobe, catheter or energy dissipating sheath. In this way, therelatively large thermal mass of the probe or catheter becomes an energydissipating surface and any temperature rise is just a few degrees C. Ingeneral, a few degrees of temperature rise is not harmful to bodytissue. In order to cause permanent damage to body tissue, such as anablation scar, it generally requires temperatures above 20° C. Theabandoned lead cap of the present invention works in identical manner bydiverting energy away from the implanted lead and its associated distalelectrodes to an energy dissipating surface that is associated with theabandoned lead cap. In a preferred embodiment, diverting circuitrycontained within the abandoned lead cap, as described in FIGS. 3 to 11,is utilized to optimize energy transfer from the lead to the EDSsurface. In summary, the frequency selective reactive element 112, whichmay comprise a capacitor 114 as shown in FIG. 5, forms a diversioncircuit such that high frequency energy is diverted away from the distalelectrodes 108 and 110 along the leads 104 and 106 to a surface that isdistant from the electrodes 108 and 110, at which point the energy isconverted to heat.

FIG. 6 describes a different way of diverting high frequency energy awayfrom the electrodes 108, 110 and accomplishing the same objective. Thegeneral diverting reactance element 112 described in FIG. 4 is shown inFIG. 6 to comprise a capacitor 114 in series with an inductor 116 toform an L-C trap circuit. For the L-C trap, there is a particularfrequency (f_(r)) at which the capacitive reactance X_(C) and theinductive reactance X_(L) are vectorally equal and opposite and tend tocancel each other out. If there are no losses in such a system, thisresults in a perfect short circuit between leads 104 and 106 at theresonant frequency. The frequency of resonance of the trap filter isgiven by the equation

${f_{r} = \frac{1}{2\;\pi\sqrt{LC}}},$wherein f_(r) is the frequency of resonance in Hertz, L is theinductance in henries, and C is the capacitance in farads.

FIG. 7 illustrates any of the aforementioned frequency dependentdiverting impedance elements 112 with the addition of series frequencyselective impeding reactances 118 and 120. The addition of seriesimpedance further impedes or blocks the flow of high frequency MRIinduced currents to the ring electrodes 108 and 110 as will be morefully described in the following drawings.

FIG. 8 is the low frequency model of FIG. 4, 5 or 6. In this regard,FIG. 8 is identical to FIG. 3, in that once again it shows theelectrical leads 104 and 106 connected to the distal ring electrodes 108and 110 of the probe or catheter 102. In the low frequency model, thefrequency reactive diverting impedance elements 112 disappear because atlow frequency their impedances approach infinity. Of course, elongatedleads in a probe or catheter are electrically and functionallyequivalent to leads used for cardiac pacemakers, implantablecardioverter defibrillators, neurostimulators and the like. For example,reference is made to U.S. Pat. No. 7,363,090, the contents of which areincorporated herein. Accordingly, any discussion herein related toprobes or catheters apply equally to leads for all active implantablemedical devices as described in FIG. 1, and vice versa. Referring onceagain to FIG. 8, this is also the low frequency model of the circuitsshown in FIG. 7. At low frequency, the frequency selective or reactivediverting component 112 tends to look like a very high or infiniteimpedance. At low frequency, the series reactive or frequency variableimpeding elements 118 and 120 tend to look like a very low impedance orshort circuit. Accordingly, they all tend to disappear as shown in FIG.8.

FIG. 9 is a high frequency model that illustrates how the distalelectrodes or rings 108 and 110 are electrically isolated at highfrequency by shorting leads 104 and 106 at location 122. As previouslymentioned, such shorting or current diverting could be accomplished by adirect short, a capacitor, a capacitive low pass filter or a seriesresonant L-C trap circuit. FIG. 9 also shows the electrodes 108 and 110as cut or disconnected and electrically isolated from the rest of thecircuit. This is because at very high frequency series impeding elements118 and 120 tend to look like a very high impedance or an open circuit.In summary, by reactive elements 112, 118 and 120 acting cooperatively,reactive element 112 diverts the high frequency energy back into energydissipating surfaces in the probe or catheter while at the same timereactive elements 118 and 120 impede the high frequency RF energy.Accordingly, in the ideal case, at high frequencies, the equivalentcircuit of FIG. 9 is achieved. Accordingly, excessive high frequency MRIRF energy cannot reach the distal ring electrodes 108, 110 and causeundesirable heating at that critical tissue interface location.

FIG. 10 shows any of the previously described diverting frequencyselective impedance elements 112 in combination with series reactancecomponents shown in the form of a pair of inductors 116 a, 116 b. It iswell known to electrical engineers that the inductive reactance in ohmsis given by the equation X_(L)=2πfL. In this case the frequency term (f)is in the numerator. Accordingly, as the frequency increases, thereactance (ohms) of the inductors also increases. When the frequency isvery high (such as 64 MHz) then the reactance in ohms becomes extremelyhigh (ideally approaches infinity and cuts off the electrodes). Byhaving a short circuit or very low impedance between the leads 104 and106, and the probe/catheter body and then, at the same time, having avery high impedance in series with the electrodes from inductors 116,this provides a very high degree of attenuation to MRI RF pulsedfrequencies thereby preventing such energy from reaching the distal ringelectrodes 108 and 110. In FIG. 10, the line-to-line selective impedanceelement 112 diverts high frequency energy back into leads 104 and 106while at the same time the series inductors 116 impede (or cut-off) highfrequency energy. When the line-to-line element 112 is a capacitor 114as shown in FIG. 5, then this forms what is known in the prior art as anL section low pass filter, wherein the capacitor 114 electricallycooperates with the inductors 116 (FIG. 10) to form a 2-element low passfilter. It will be obvious to those skilled in the art that FIG. 5describes a single element (capacitor) low pass filter, and that FIG. 10describes a 2-element or L-section low pass filter. Moreover, any numberof inductor and capacitor combinations can be used for low pass filters,including 3-element Pi or T circuits, LL, 5-element or even “n” elementfilters.

FIG. 11 offers an even greater performance improvement over thatpreviously described in FIG. 10. In FIG. 11, modified frequencyselective impeding elements each incorporate a parallel resonantinductor 116 and capacitor 114 which is also known in the industry as abandstop filter 123. The L-C components for each of the reactiveelements are carefully chosen such that each of the bandstop filters 123is resonant, for example, at the pulsed resonant frequency of an MRIscanner. For common hydrogen scanners, the pulsed resonant frequency ofan MR scanner is given by the Lamor equation wherein the RF pulsedfrequency in megahertz is equal to 42.56 times the static fieldstrength. For example, for a popular 1.5 Tesla scanner, the RF pulsedfrequency is 64 MHz. Common MR scanners that are either in use or indevelopment today along with their RF pulsed frequencies include: 0.5Tesla—21 MHz; 1.5 Tesla—64 MHz; 3 Tesla—128 MHz; 4 Tesla—170 MHz; 5Tesla—213 MHz; 7 Tesla—300 MHz; 8 Tesla—340 MHz; and 9.4 Tesla—400 MHz.When the bandstop filters 123 are resonant at any one of these RF pulsedfrequencies, then these elements tend to look like an open circuit whichimpedes the flow of RF current to distal electrodes. When compatibilitywith different types of MR scanners is required, for example, 1.5, 3 and5 Tesla, then three separate bandstop filter elements in series maycomprise the reactive element 118 (FIG. 7), and three separate bandstopfilter elements in series may comprise the reactive element 120 (FIG.7). Each of these would have their L and C components carefully selectedso that they would be resonant at different frequencies. For example, inthe case of MR scanners operating at 1.5, 3 and 5 Tesla, the threebandstop filters comprising the reactive element 118 as well as thethree bandstop filters comprising the reactive element 120 would beresonant respectively at 64 MHz, at 128 MHz, and at 170 MHz. Theresonant frequencies of the bandstop filter elements could also beselected such that they are resonant at the operating frequency of otheremitters that the patient may encounter such as diathermy and the like.The use of bandstop filters 123 is more thoroughly described in U.S.Pat. No. 7,363,090; US 2007/0112398 A1; US 2007/0288058; US 2008/0071313A1; US 2008/0049376 A1; US 2008/0161886 A1; US 2008/0132987 A1 and US2008/0116997 A1, the contents of which are incorporated herein.

Referring now to FIG. 12, a prior art active implantable medical device(AIMD) 100 is illustrated. In general, the AIMD 100 could, for example,be a cardiac pacemaker 100C which is enclosed by a titanium or stainlesssteel conductive housing 124. The conductive housing 124 is hermeticallysealed and contains a battery and electronic circuits, however, there isa point where conductors 126 a, 126 b, 126 c and 126 d must ingress andegress in non-conductive relationship relative to the housing 124. Thisis accomplished by providing a hermetic terminal assembly 128. Hermeticterminal assemblies 128 are well known and generally consist of aferrule 129 which is laser welded to the titanium housing 124 of theAIMD 100C. In FIG. 12, four conductive leadwires 126 a-126 d are shownfor connection to a corresponding number of leads, such as theillustrative bipolar leads 104 and 106 shown for coupling to theconnector receptacles 130. In this configuration, the four leads coupledrespectively to the conductors 126 a-126 d comprise a typical dualchamber bipolar cardiac pacemaker. It should be noted that each of thebipolar leads 104 and 106 have a pair of leads associated with them.These are known as bipolar electrodes wherein one wire is routed to thetip electrode and the other is routed to the ring electrode in locations108 and 110.

Connectors 132 are commonly known as IS-1 connectors and are designed toplug into mating receptacles 130 on a header block 134 mounted on thepacemaker housing 124. These are low voltage (pacemaker) lead connectorscovered by an International Standards Organization (ISO) standard IS-1.Higher voltage devices, such as implantable cardioverter defibrillators,are covered by a standard known as the ISO DF-1. A newer standard hadbeen published that integrates both high voltage and low voltageconnectors into a new miniature quadpolar connector series known as theISO IS-4 standard. Leads plugged into these connectors are typicallyrouted in a pacemaker or ICD application into the right ventricle andright atrium of the heart 136.

In the following description, functionally equivalent elements shown invarious embodiments will often be referred to utilizing the samereference number.

Referring once again to the prior art AIMD 100, such as the cardiacpacemaker 100C in FIG. 12, generally such AIMDs have primary batteriesthat have a limited lifetime. It is very common in the art, since theAIMD is laser welded and hermetically sealed, that when batteryreplacement is due, the entire AIMD is replaced. If there is nothingwrong with the implanted leads 104 and 106, they are generally reused.However, in many cases, there are lead defects, poor impedancecharacteristics, or even abrasions in a lead that cause the physician toabandon them and instead, insert new leads. It is a relatively easymatter to insert new leads endocardially in parallel with the existingleads. These new leads are then connected to a new AIMD, such as a newcardiac pacemaker. Accordingly, the previous leads are left in the body,which will be shown, are problematic for MRI.

FIG. 13 illustrates a typical dual chamber IS-1 header block 134 whichis attached to a partial cutaway view of a prior art cardiac pacemaker100C.

FIG. 14 is an alternative AIMD header block 134 which illustrates fourconnector ports. This header block is attached to a partial cutaway viewof a prior art implantable cardioverter defibrillator (ICD) 100I.

FIG. 15 illustrates a drawing of the new inline quadripolar IS-4 (138)lead with four electrodes C₁ through C₄ which incorporates both lowvoltage and high voltage circuits all in one.

FIG. 16 illustrates a typical lead system for an ICD. The leads of FIG.16 are designed to match up with the header port 134 as shown in FIG.14. Referring once again to FIG. 16, one can see that there are two lowvoltage IS-1 leads 132 and 132′. Typically, one of these would be routedto the right atrium and the other would be routed to the rightventricle. There are also two high voltage shocking leads DF-1 (140).Typically, one DF-1 electrode would be routed to the right ventricle andthe other would be routed to the inside of the superior vena cava.

FIG. 17 illustrates a prior art single chamber bipolar AIMD 100C andleads 104 and 106 with a distal tip electrode 108 and a ring electrode110 typically as used with a cardiac pacemaker 100C. Should the patientbe exposed to the fields of an MRI scanner or other powerful emitterused during a medical diagnostic procedure, currents that are directlyinduced in the leads 104, 106 can cause heating by I²R losses in theleads or by heating caused by RF current flowing from the tip and ringelectrodes 108, 110 into body tissue. If these induced RF currentsbecome excessive, the associated heating can cause damage or evendestructive ablation to body tissue 136.

FIG. 18 illustrates a single chamber bipolar cardiac pacemaker 100C, andleads 104 and 106 having distal tip 138 and distal ring electrode 108.This is a spiral wound (coaxial) lead system where the tip electrodelead 104 is wrapped around the ring electrode lead 106. Thecharacteristic impedance of this lead type usually has an inductivecomponent. There are other types of pacemaker lead systems in whichthese two leads that lay parallel to one another (known as a bifilarlead system), which are not shown.

FIG. 19 is an enlarged schematic illustration of the area “19-19” inFIG. 18. In the area of the distal tip electrode 108 and the ringelectrode 110, bandstop filters 123 a, 123 b have been placed in serieswith each of the respective ring and tip circuits. The ring circuit lead104 has been drawn straight instead of coiled for simplicity. Thebandstop filters 123 are tuned such that, at an MRI pulsed RF frequency,a high impedance will be presented thereby reducing or stopping the flowof undesirable MRI induced RF current from the electrodes 108 and 110into body tissues.

FIG. 20 is a line drawing of a human heart 136 with cardiac pacemakerdual chamber bipolar leads 104, 106 shown in the right ventricle 142 andthe right atrium 144. FIG. 20 is taken from slide number 3 from aPowerPoint presentation given at The 28^(th) Annual Scientific Sessionsof the Heart Rhythm Society by Dr. Bruce L. Wilkoff, M. D. of theCleveland Clinic Foundation. This article was given in Session 113 onFriday, May 11, 2007 and was entitled, ICD LEAD EXTRACTION OF INFECTEDAND/OR REDUNDANT LEADS. These slides are incorporated herein byreference and will be referred to again simply as the Wilkoff reference.In FIG. 20, one can see multiple leads 104, 106 extending from an activeimplantable medical device 100C (such as a cardiac pacemaker or thelike) coupled to associated electrodes, one of which comprises thedistal tip ventricular electrode 108 located in the right ventricular142 apex. The dark shaded areas in FIG. 20 show the experience of theCleveland Clinic and Dr. Wilkoff (who is a specialist in leadextraction), where extreme tissue overgrowth and vegetation tends tooccur. There are numerous cases of extracted leads where both the tipand ring electrodes have been overgrown and encapsulated by tissue.Referring once again to FIG. 20, one can see tip electrode 108, which islocated in the right ventricular apex 142. The shaded area encasing thiselectrode 108 shows that this area tends to become encapsulated by bodytissue. A distal tip electrode 108′ in the right atrium 144 maysimilarly be overgrown and encapsulated by tissue, as shown by theencasing shaded area. There are other areas in the superior vena cavaand venous system where leads tend to be encapsulated by body tissue agreat percentage of the time. These are shown as areas 146 and 148.

Referring once again to FIG. 20, as previously mentioned, it is veryimportant that if this lead system is abandoned that it not overheatduring MRI procedures particularly at or near the distal tip and ringelectrodes 108, 110. If either or both the distal tip and ring electrodebecome overgrown by body tissue, excessive overheating can causescarring, burning or necrosis of said tissues. Often times when thedevice such as a pacemaker 100C shown in FIG. 20 is changed out, forexample, due to low battery life and a new pacemaker is installed, thephysician may decide to install new leads at the same time. Leads arealso abandoned for other reasons, such as a dislodged or a highimpedance threshold. Sometimes over the course of a patient life-time,the distal tip electrode-to-tissue interface increases in impedance.This means that the new pacemaker would have to pulse at a very highvoltage output level which would quickly deplete its battery life. Thisis yet another example of why a physician would choose to insert newleads. Sometimes the old leads are simply extracted. However, this is avery complicated surgical procedure which does involve risks to thepatient. Fortunately, there is plenty of room in the venous system andin the tricuspid valve to place additional leads through the samepathway. The physician may also choose to implant the pacemaker on theother side. For example, if the original pacemaker was in the rightpectoral region, the physician may remove that pacemaker and choose toinstall the new pacemaker in the left pectoral region using a differentpart of the venous system to gain lead access. In either case, theabandoned leads can be very problematic during an MRI procedure.

In general, prior art abandoned leads are capped with a silicone cap attheir proximal connector points so that body fluids will not enter intothe lead system, cause infections and the like. However, it has beenshown in the literature that the distal electrodes of abandoned leadsare at high risk to heat up during MRI procedures. Accordingly, theabandoned lead cap of the present invention when associated with anenergy dissipation surface is very useful when placed at or near theproximal electrical contact after a pacemaker is removed and its leadsare disconnected (abandoned). Referring back to the article by Dr. BruceWilkoff, attention is drawn to slide number 2, which is an example of alead extraction showing both a distal tip electrode and a distal ringwhich have been heavily overgrown and encapsulated by body tissue.

FIG. 21 is a line drawing showing lead systems with various types ofelectrode tips 108. For instance, electrode tip 108 a is a passivefixation right atrium pacing lead, electrode tip 108 b is an activefixation bi-ventricular pacing lead, and electrode tip 108 c is aventricle defibrillation lead.

If the leads and their associated electrodes as described in FIG. 21were abandoned, it would be important that each of the leads beconnected to an abandoned lead cap(s) that are associated with an energydissipating surface. In a preferred embodiment, a passive componentfrequency selective diverter element would be used such that highfrequency RF energy from the lead would be coupled to the EDS surface.The word passive is very important in this context. Active electroniccircuits, which are defined as those that require power, do not operatevery well under very high amplitude electromagnetic field conditions.Active electronic filters, which generally are made from microelectronicchips, have very low dynamic range. Extremely high fields inside an MRIchamber would tend to saturate such filters and make them becomenonlinear and ineffective. Accordingly, frequency selective networks arepreferably realized using non-ferromagnetic passive electroniccomponents. In general, this means that the frequency selectivecomponents for both diverters and impeders preferably consist ofcapacitors, inductors, and resistors in various combinations. Passiveelectronic components are capable of handling very high power levelswithout changing their characteristics or saturating. Moreover, theinductor elements are preferably made from materials that are notferromagnetic. The reason for this is that MRI machines have a verypowerful main static magnetic field (B₀). This powerful static magneticfield tends to saturate ferrite elements and would thereby changedramatically the value of the inductance component. Accordingly, in thepresent invention, the inductor elements are preferably fabricatedwithout the use of ferrites, nickel, iron, cobalt or other similarferromagnetic materials that are commonly used in general electroniccircuit applications. In accordance with the present invention, energydiversion circuits were previously shown in FIGS. 4, 5 and 6. Activeelectronic filters could also be used as diverter circuits, however, aspreviously mentioned, their dynamic range would be limited and theywould tend not to work very well in the presence of very powerful RFfields.

FIG. 22 illustrates an exemplary energy diverter 112 (as previouslyshown described in connection with FIGS. 4, 5 and 6) comprising, forexample, a capacitor 114 (as previously shown and described herein) withnonlinear circuit elements such as diodes 150 a and 150 b placed inparallel therewith. These diodes 150 a, 150 b are oriented in aback-to-back configuration. The diode elements 150 a, 150 b, asillustrated in FIG. 22, can be placed in parallel with each other, andwith any of the frequency selective circuit elements shown in FIGS. 4through 11. For example, referring to FIG. 5, the diode elements couldbe placed in parallel with the capacitive element 114. Referring to FIG.10, two diode elements could also be placed in parallel with each of theinductor elements 116 a and 116 b. Back-to-back diodes are one form of atransient voltage suppressor 152.

Transient voltage suppressors (TVS) are well known in the prior art forproviding over voltage circuit protection. They are sold under varioustrade names including the name Transorb. The diodes 150 a, 150 b canalso be pin diodes. Since automatic external defibrillators (AEDs) havebecome very popular in the patient environment, the diverter circuits ofabandoned lead caps 154 must be able to withstand very high pulsedcurrents. These pulse currents can range anywhere from 1 to 8 amps. Thepassive frequency selective components used in accordance with thepresent invention are typically very small in size. In order for aninductor element L to be able to handle 1 to 8 amps, it would have to beexceedingly large. However, by using physically small diode elements 150a and 150 b, one can have the circuits switched to a different state.That is, when a high voltage, such as that from an AED appears, thediodes would forward bias thereby temporarily shorting out the diverter112. Thereby the correspondingly high AED induced currents would bediverted away from the relatively sensitive (small) passive elements L(116) and C (114) of the diverter element 112 in such a way that theynot be harmed.

FIG. 23 is a schematic diagram that is very similar to FIG. 22 exceptthat it shows that the diverter element 112 could also be an impederelement 118. In either case, a transient voltage suppressor (TVS) 152 isshown in parallel. The TVS is inclusive of the back-to-back diodes 150 aand 150 b previously illustrated in FIG. 22. The transient voltagesuppressor 152 includes all types of transient voltage suppressors,diode arrays, Transorbs, etc.

FIG. 24 shows a unipolar lead system 104 attached to a novel abandonedlead cap 154 of the present invention. Associated with the abandonedlead cap 154 is an energy dissipating surface 160. Also shown is afrequency selective diverter circuit 112 which is connected between thelead 104 and the EDS surface 160. A unipolar lead system is shown forsimplicity. It will be obvious to those skilled in the art that anynumber of lead wires 104 could be used. In FIG. 20, one will see thatthis system involves an AIMD 100C attached to bipolar leads 104, 106 toa human heart 136. At the distal tip or distal end of lead wire 104, 106is an optional bandstop filter 123 as illustrated in FIGS. 11 and 19.The optional bandstop filter 123, which is usually located at or nearthe distal electrode 108, is more thoroughly described in U.S. Pat. No.7,363,090 the contents of which are incorporated herein. As shown, theimplanted lead has inductive 162 and resistive 156 properties along itslength (it may also have capacitive properties as well). The total orequivalent inductive reactance of the lead in ohms is given by theformula +jωL. As mentioned, a distal electrode bandstop filter 123 mayor may not be present. The equivalent inductance 162 and resistance 156of the lead system also includes the impedance of any tissue returnpath. It should be noted that the present invention applies to any typeof abandoned AIMD lead. For example, there are certain neurostimulatorapplications involving a number of distal electrodes that all havereturn paths through body tissue from a distal electrode 108 all the wayto the abandoned lead cap. One of the best ways to actually determinethe characteristic lead impedance, including its inductive 162,capacitive, and resistive 156 properties, is through human body modelingusing software such as SAMCAD. Using SAMCAD, one can calculate theelectric field vectors all along the lead trajectory. One can thencalculate the induced energy into the implanted leads and theircharacteristic impedances. Referring once again to FIG. 24 one can seethat on the interior of the abandoned lead cap 154 there are frequencyselective components 112. These frequency selective elements 112 canconsist of various arrangements of capacitors 114, inductors 116 andresistors 158 or even short circuits as will be more fully described inFIGS. 25 through 34.

FIG. 25 is very similar to FIG. 24 except that the diverter element 112has been combined with an impeder element 118. Diverter circuits 112were previously described in FIGS. 4 through 6. Impeder elements 118were previously described in association with diverter elements in FIGS.7 through 11. The lead cap 154 of the present invention can therefore becombined with any number of combinations of diverter elements 112 andimpeder elements 118.

FIG. 26 shows an implanted unipolar lead system 104 which is identicalto that previously described in FIG. 24. In FIG. 26, the diverterelement 112 is shown as a capacitor 114. The capacitor in this case actsas a high pass filter in that it allows high frequency MRI RF energy tobe diverted from the lead 104 to the EDS surface 160 of the abandonedlead cap 154. The capacitor 114 creates a −j capacitive reactance whichtends to cancel the +j inductive reactance 162 that is associatedtypically with an implanted lead. This facilitates maximum energytransfer to the EDS surface 160 as has been previously described.

FIG. 27 is identical to FIG. 26 except that a resistor element 158 hasbeen added in series with the capacitor element 114. Resistor element114 can be part of the capacitor's 114 equivalent series resistance(ESR). Alternatively, the resistor 158 could be a separate discreteresistor, such as a chip resistor. In a particularly preferredembodiment, the value of the resistance 158 would be equal to thecharacteristic or equivalent resistance of the lead 156. Again,according to Thevenin's Theorem, this would facilitate maximum energytransfer to the EDS surface 160.

FIG. 28 is very similar to FIG. 24 except the diverter element 112 hasbeen replaced with a short circuit. The short circuit directly connectsthe implanted lead 104 to the EDS surface 160 of the abandoned lead cap154. Although not optimal, for energy efficiency, this simple approachstill directs a great deal of MRI RF induced energy from the lead 104 tothe EDS surface 160 in accordance with the present invention.

FIG. 29 is also very similar to FIG. 24 except that the diverter element112 has been replaced with a simple resistor element 158. In order toaccomplish maximum energy transfer in accordance with Thevenin'sTheorem, diverter resistor 158 would be equal to the characteristicresistance 156 of the implanted lead. The short circuit illustrated inFIG. 28 and the resistor 158 illustrated in FIG. 29 are not optimal formaximum energy transfer to the EDS surface 160. However, in certainabandoned lead situations, particularly those that are near the MRI boreor ISO center, these would be adequate.

FIG. 30 is very similar to FIG. 24 except that the diverter element 112consists of an inductor 116 in series with a capacitor 114. This isknown as an L-C trap filter as was previously described in FIG. 6. Inthis case, the inductor element 116 and the capacitor element 114 havebeen designed to be resonant at the pulsed RF frequency of the MRIequipment. Therefore, at this selected frequency, this forms an RF shortto the EDS surface 160 of the abandoned lead cap 154.

As shown in FIG. 31, a series resistance 158 could be added in serieswith the L-C trap filter of FIG. 30 consisting of the inductor 116 andthe capacitor 114. The resistor 158 further optimizes energy transferfrom lead wire 104. The resistance 158 is also used to control the Q ofthe resonant L-C trap filter and its associated resonant bandwidth.

FIG. 32 is very similar to FIG. 25 showing both a diverter element 112and impeder element 118. The diverter element 112 as shown in FIG. 32could be any of the diverters as previously described in FIGS. 26through 31. In this case, the impeder element is a bandstop filter 123as previously described in FIG. 11.

FIG. 33 is very similar to FIG. 32 except that the impeder element 118is a simple inductor 116 which was previously described in FIG. 10.

FIG. 34 combines various diverter and impeder elements all into onecircuit inside of a novel abandoned lead cap 154. Shown is a divertercapacitor 114 in series with a bandstop filter impeder 123 which is thenconnected to an RLC trap filter consisting of resistor 158, inductor 116and capacitor 114 connected to the energy dissipating surface 160. FIG.34 shows a bandstop filter 123 which is useful when one wishes to impedethe flow of currents in implanted lead at certain frequencies such aselectrocautery frequencies, while at the same time diverting MRI highfrequency induced RF energy to the EDS surface 160. In other words, onecould prevent excess electrocautery current from flowing through adistal electrode while at the same time diverting high frequency MRIenergy to the EDS surface. The circuit shown in FIG. 34, inside of theAIMD abandoned lead cap 154, is illustrative of just one combination.Any combination of the circuits described in FIGS. 3 through 11 may beembodied and in any combinations.

FIG. 35 is a patient front view representative of an X-ray tracing of animplanted cardiac pacemaker. The pacemaker 100 c is shown installed in apectoral pocket, which can either be left pectoral (as shown) or rightpectoral (not shown). There can be one or more turns of excess lead 104a that's coiled up in the pectoral pocket and then the lead 104 isrouted endrocardially down through the superior vena cava into a cardiacchambers 136 as shown. A loop area shown by the checker pattern 164 isformed between the distal tip electrode 108 all along the lead 104 tothe AIMD 100 c and then through a multi-path tissue return path shown166 as dashed lines from the AIMD 100 c to the distal tip electrode 108in the heart. MRI low frequency gradient fields couple into this loop164 by Faraday's Law of Induction. In general, Faraday's Law states thata voltage induced in this loop 164 is directly proportionate to the areaof the loop times the rate of change of the magnetic field in Teslas persecond. A worse case coupling situation occurs when the field isorthogonal to the loop area 164. Current will flow in the lead 104unless the lead is opened up (switched open). It is highly undesirablethat this low frequency MRI gradient induced current flow into cardiactissues as this could directly induce cardiac arrhythmias. It is alsoundesirable if this current should flow into the AIMD electronics as itcould either interfere with AIMD electronics (EMI) or it could lead togradient rectification. In the art, direct cardiac or tissue stimulationis known as Gradient STIM.

FIG. 36 is another illustration of the unipolar lead system of FIG. 26.In this case, the diverter element 112 features a capacitive element 114whose capacitive reactance is given by the equation −j/ωC. In apreferred embodiment, the inductance of the implanted lead would firstbe modeled, calculated or measured. Therefore, the value of capacitancecould be tuned or selected such that −j/ωC is equal and opposite to thelead 104 characteristic inductive reactance +jωL. In this case, thereactances cancel each other so that one gets maximal energy transfer tothe abandoned lead cap 154 energy dissipating surface 160. As previouslydescribed, the capacitor's equivalent series resistance (ESR) could becontrolled or a discrete resistance approximately equal to thecharacteristic resistance of the implanted lead could be added in seriesin order to further maximize energy transfer from the implanted leadsystem 104 to the EDS surface 160.

FIG. 37 is the high frequency model of the circuit illustrated in FIG.36. In this case, at high frequencies, such as MRI RF pulsedfrequencies, the capacitor 114 is a very low impedance which effectivelyappears as a short circuit. This has the desirable effect of pulling ordiverting high frequency energy on the lead 104 through the lowimpedance of the capacitor 114 to the energy dissipating surface 160.The capacitor 114 can be modeled by a switch 168 that is shown closed orshorted to the EDS surface 160. For maximal energy transfer from thelead 104, the capacitive reactance of capacitor 114 has a −j vectorwhich tends to cancel the +j vector that is associated with thecharacteristic inductive reactance of an implanted lead 104.

FIG. 38 is the low frequency model of the circuit previously illustratedin FIG. 36. In this case, at low frequencies, the capacitor 114 appearsas a very high impedance which effectively appears electrically as anopen circuit. As previously mentioned, this has the desirable effect ofpreventing gradient currents from flowing in the implanted lead and theassociated loop through body tissue and AIMD electronics. In otherwords, the diverter element capacitor 114 that was illustrated in FIG.36 performs two very important functions. The first function is that itdiverts high frequency RF energy that is induced on the lead fromexposure to high power RF fields such as occur during magnetic resonanceimaging. In addition, the capacitor looks like a very high impedancewhen energy is coupled to the lead 104 from low frequency MRI gradientfields which are typically below 4 kHz. In this case, the capacitorlooks like an open circuit or open switch that prevents these gradientcurrents from flowing in an implanted lead loop area 164. It has beenshown that gradient induced currents can directly stimulate tissue. Suchcurrents have been known to be captured by the heart as dangerously highrepetition rates or directly induce pain into the spinal column or haveother deleterious effects. In the case of capturing the heart at thehigh rate, this can induce a ventricular arrhythmia that can even belife threatening. Referring once again to FIG. 36, the capacitor 114 canbe combined with a series resistance element or it could even bereplaced by a series RLC trap filter or other diverter circuit aspreviously described herein. All of these would have the desired effectof shutting high frequency RF energy to the abandoned lead cap EDScircuit while at the same time preventing gradient current flow in theimplanted lead loop.

FIG. 39 illustrates a typical cardiac IS-1 proximal end connector 132.It is attached to the end of an implanted lead 104 as shown. Shown aretwo contact surfaces 170 and 172. Contact surface 170 typically connectsto a distal tip electrode 108. The contact surface 172 typicallyconnects to ring electrode 110.

FIG. 40 illustrates a cross-section of a novel abandoned lead cap 154 ofthe present invention which is suitable for termination of an IS-1 leadmale connector 132. Shown are two set screws 174 which are used to affixthe lead 104 contact surfaces 170 and 172 firmly in place. Inalternative embodiments, using IS-1 or alternative connectors, there maybe a single-set screw where the other electrical connection is made byan o-ring spring contact-like mechanism. There is an insulativedielectric material 176 which can be of typical medical grade plastic orthe like. The conductive tip 170 and ring 172 of the IS-1 lead connector132 is shown connected to a metal plate 178. The metal plate 178 can beslightly embedded within the insulator 176 to form a parasiticcapacitance between the metal plate 178 and the surrounding body tissuein the abandoned lead cap area. Set screws 174 are threaded into metalblocks 180 so they make both electrical and mechanical contact to theelectrical contact areas 170 and 172 with a proximal IS-1 connector 132.The metal blocks 180 are electrically connected via the electricalconnection material 182 to the metal plate 178. The amount ofcapacitance is determined by the dielectric thickness “d” and thesurface area of the metal plate 178 and also the dielectric constant ofthe insulating material and surround human tissues. As previouslystated, the capacitive reactance in ohms is given by the equation

${Xc} = {\frac{1}{2\;\pi\;{fC}}.}$Therefore, at high frequency, such as 64 MHz, the abandoned lead cap, inthe area of metal plate 178, becomes an energy dissipating surface 160.

FIG. 41 is the same as FIG. 40 except that the IS-1 connector 132 hasbeen removed. In other words, FIG. 41 is the abandoned lead cap 154 thatwould be kept in hospital inventory prior to its insertion on the tip ofan abandoned IS-1 lead connector 132.

FIG. 42 is an isometric view of the novel abandoned lead cap of thepresent invention as previously illustrated in FIGS. 40 and 41. FIG. 42is for termination of a bipolar lead in that it only has one IS-1 port184. Also shown are suture holes 186. These are to be used by theimplanting physician to run a suture so that the novel abandoned leadcap 154 of FIG. 42 can be affixed to surrounding body tissues, bone orthe like. Shown is an RFID tag 188 which consists of an RFID microchip190 disposed on substrate 192. Associated with the RFID chip is an RFIDantenna 194. RFID tags are well known in the prior art and take on avariety of shapes including circular antennas, spiral antennas, solenoidantennas fielded dipoles or a circuit trace antenna as illustrated. TheRFID tag 188 enables a means of quick electronic identification byemergency room personnel, a radiologist, or an implanting physician thatthe patient indeed has an abandoned lead and also that the abandonedlead has a cap with an EDS surface that is MRI compatible, for example,with a 1.5 Tesla system. It should be noted that just because anabandoned lead is compatible with a 1.5 Tesla system does not mean it issafe, for example, in other MRI systems such as 3.0 Tesla systems. US2006/0212096 and U.S. Ser. Nos. 12/566,490 are incorporated herein byreference. The RFID tag 188 can be incorporated with any novel abandonedlead cap 154 that is associated with an energy dissipating surface 160.

FIG. 43 is an isometric view which is identical to FIG. 42 except thatit has two IS-1 ports 184 a and 184 b. It should be noted in FIG. 43that the tip and ring connectors 170 and 172 are both shorted to thecommon metal plate 178′. In other words, at the point of lead pointtermination, all four of the lead wires (two bipolar leads) 104 are allshorted together and connected to metal plate 178′. In some implantedlead configurations, there is a concern that the common metal plate 178′could act as a common reference point. In other words, if there weresubstantial RF energy be interjected into lead port 184 a, but not onlead port 184 b, then via the common metal plate, energy could bereflected from one implanted lead back down to another. As will be seenin subsequent figures, such as FIG. 45 and on, variable impedancediverter 112 and impeder 118 elements can be used to balance this so itdoes not happen.

FIG. 44 illustrates an alternative embodiment showing four IS-1 or DF-1ports 184 a through 184 d. For example, this could be used to terminatethe leads of an implantable defibrillator lead system as previouslyillustrated in FIG. 16. In this particular case, two of the ports wouldbe IS-1 (132) compatible and two of the other ports would be DF-1 (140)compatible.

FIG. 45 is a schematic diagram showing the unipolar abandoned lead cap154 of FIGS. 40, 41 and 42. Shown is a general diverter element 112,which in the case of FIGS. 40, 41, 42, 43 and 44, are capacitor elements114 as previously described in connection with FIG. 5.

FIG. 46 is a modification of FIG. 41 showing both an internal metalplate 178 and an external metal plate EDS surface 160. This forms ahigher value and a more precise capacitance 114 between metal plate 178and the energy dissipating surface 160 as shown. The capacitance isformed by the overlapping area of plate 178 and the EDS surface plate160. The capacitance is effected not only be the overlapped area, butalso by the separation thickness between the two plates and thedielectric material of the plastic or insulating material that isdisposed between the two plates.

FIG. 47 is yet another modification of the novel abandoned lead cap 154as illustrated in FIG. 46 wherein a discrete capacitive element 114 hasbeen electrically connected between the metal plate 178 and the EDSsurface 160. Referring back to FIG. 47, the dielectric constant of thecapacitor 114 would be relatively high. In this way, a relatively highvalue of capacitance could be disposed as diverter elements between thedistal tip and ring electrodes 170, 172 and the energy dissipatingsurface 160 as shown. The schematic diagram, as previously illustratedin FIG. 45, would still apply except that the value of capacitance Cwould be much larger. Accordingly, the amount of RF energy dissipated inFIG. 47 would be desirably much greater. Also, the value of capacitance144 could be carefully selected so that the capacitive reactanceeffectively cancels the inductive reactance of the implanted lead systemfor maximum energy transfer. The capacitor 114 as shown could be a priorart monolithic ceramic chip capacitor (MLCC), a chip film capacitor, anelectrolytic capacitor, a tantalum capacitor or any other type ofdiscrete capacitor technology.

FIG. 48 is an isometric view similar to the unipolar lead cap of FIG.47. However, in this case, two different capacitors 114 a and 114 b areused to make contact to metal plates 178 a and 178 b respectively, whichare in turn both connected to exterior energy dissipation surface 160.Not that plates 178 a and 178 b are electrically insulated from eachother. For example, in a pacemaker application, this would be desirablein the case where the implanted lead has a different inductance for thetip circuit as opposed to the ring circuit. Since these are usuallycoiled and are of different diameters, this would typically be the case.

FIG. 49 illustrates yet another embodiment wherein the tip 170 and ring172 are both shorted together to metal plate 178 and that an L-Cresonant circuit (FIG. 6) is connected between metal plate 178 andexterior energy dissipation surface 160. As previously mentioned, thevalues of L and C can be selected such that this trap circuit isresonant, for example, at the resonant frequency of a 1.5 Tesla MRIsystem (64 MHz). Referring once again to FIG. 49, one notes that theenergy dissipating surface 160 is disposed on the outside of theabandoned lead cap 154 where it is in direct contact with surroundingbody tissue. This makes for a preferred embodiment where a higher amountof energy transfer will occur.

FIG. 50 illustrates that circuit elements 118 and 112 can be any ofthose as previously illustrated in FIGS. 2 through 11. For example,impeder element 118 could be one or more series inductors 116 asillustrated in FIG. 10. In addition, impeder element 118 could be one ormore bandstop filters 123 as illustrated in FIG. 11. Frequency selectivediverter element 112 could be a simple capacitor 114 as illustrated inFIG. 5 or could be an L-C trap as illustrated in FIG. 6. It could alsobe a short circuit as illustrated in FIG. 9.

FIG. 51 is very similar to FIG. 40 except that metal plate 178 has beenremoved and the EDS surface 160 is on the outside of the insulatinghousing 154. One can see that the EDS surface 160 is now in directcontact with body fluids and tissues. The IS-1 lead connector 132 isshown inserted into the metal holding blocks 180. Once the set screws174 are torqued properly, the tip 170 and ring 172 circuits will be bothelectrically shorted together to the metal plate 178, 160. This iselectrically equivalent to the short circuit as illustrated in schematicFIGS. 9 and 28.

FIG. 52 is very similar to FIG. 41 except that metal plate 178 has beendivided into two electrically isolated plates 178 a and 178 b. Thisforms two different values of capacitance C₁ and C₂ to external EDSsurface 160. By varying the effective area of metal plates 178 a and 178b, one can control the amount of capacitance that is formed to theenergy dissipating surface 160.

FIG. 53 is electrically the same as FIG. 47 except that the capacitor114 has been placed in a novel hermetic package 196. At both ends arehermetic seals 198 and 198′ as shown. It will be obvious to thoseskilled in the art that these could also be glass or gold brazed aluminaor equivalent hermetic seals. In this way, the capacitor 114 isprotected from intrusion by body fluid. Any of the component systemsillustrated in FIGS. 2 through 11 could be similarly enclosed inhermetic housings. U.S. Ser. No. 12/607,234 is incorporated by referenceherein.

FIG. 54 is a modified abandoned lead cap 154 of the present inventionincluding two springs 200 and 200′ and a locking mechanism 202. Thiseliminates the need for the set screws 174 as illustrated in previousdrawings. Another alternative is to use one spring 200 and a set screw174 as an additional locking mechanism 202. A conductive metal energydissipating surface 160 is disposed adjacent to a ceramic discoidalfeedthrough capacitor 114′ that makes connection between the metallichousing of the end cap 204 and the energy dissipating surface 160. Thisforms the equivalent circuit of FIG. 45.

Referring once again to FIG. 54, one can see that there is a threadedlocking system 206. Locking mechanism 206 is first guided onto the lead104. The lead is then inserted so it seats properly against springsurface 200 and 200′. The threaded bushing 206 is slid down and firmlyscrewed in place. This forms a barrier so that body fluid and/ormoisture cannot penetrate into the interior spaces or run down into thelead itself.

FIGS. 55 and 56 illustrate the same abandoned lead cap 154 principle asillustrated in FIG. 43 except that it is much more streamlined and has athinner shape for better patient comfort. One can see that there is atitanium or stainless steel housing 160 which forms a high surface areaenergy dissipating surface EDS surface. The titanium EDS surface 160could have a laser welded interior lid such that it formed ahermetically sealed compartment. This would be convenient for thelocation of passive electronic components which would form eitherdiverter elements 112 or impeder elements 118 of the present invention.In the case where such a hermetically sealed compartment was formed,there would be a hermetic seal (not shown) to pass leads through innon-conductive relation for attachment to the connector blocks 180′.Referring once again to the novel EDS surface shapes 160 as illustratedin FIGS. 55 and 56, one should note that these surfaces in titaniumGrades-5 and 23 have been optimized to have a mild forming radius. Ingeneral, it is very difficult to form these high resistivity grades oftitanium. They do not lend themselves well for stamping, pressing orother forming operations. Radius 207 is illustrated in FIGS. 55 and 56to show that a gradual curve is formed which is about the maximum thatone can shape Grade-5 or 23 titanium. Also shown are optional metallictabs 186′ for placement of sutures during implant operations. Ingeneral, the metallic housing 160 would ideally be of a titanium Grade-5or Grade-23 alloy such that MRI induced eddy current heating would beminimized. Another acceptable alloy would be 6AL4V (this is Grade 5).Grade 23, which is more pure, is preferred. Grade 9 is another type ofmaterial that is commonly used in the dental industry. Other gradesinclude Grade 1, Grade 2, Grade 5 and Grade 23. It is a feature of thepresent invention that an optimal grade of titanium shall be used; suchto minimize localized eddy current heating such as may be induced by themagnetic resonant frequency gradient fields. In general, the higher theelectrical resistivity of the material that forms the EDS surface 160the better. For example, pure titanium has an electrical resistivity of35 microohm-centimeters. Grade-1 titanium, which is very typically usedin prior art AIMD housing is 45 microohm-centimeters. Grade-5 titaniumis 178 microohm-centimeters and Grade-23 titanium is 168microohm-centimeters. This is why the use of grades of titanium arerelatively high in electrical resistivity are highly preferred. Thepowerful electromagnetic fields from MRI can directly induce eddycurrents into metal plates or housing. Accordingly, it is a feature ofthe present invention that high resistivity materials are preferred suchthat eddy current heating is minimized. In addition to titanium, thereare grades of stainless steel, which tend to have moderately highelectrical resistivities. These are not preferred, but are acceptable toalso minimize eddy current losses. This includes alloy 304 stainlesssteel, which is 72 microohm-centimeters; 316 stainless steel, which is74 microohm-centimeters; and a type of stainless steel alloy known asHaynes 25 which is 88.6 microohm-centimeters.

FIG. 57 is an illustration of a three-port novel abandoned lead cap 154of the present invention. One can see that there is a single low voltageconnector port 184 b for a tip and ring type of lead and two DF ports184 a and 184 c for high voltage. All of these are electricallyconnected to EDS surface 160.

FIG. 58 illustrates the use of novel slots 208 in the EDS plate 160.These slots can be of any number and of any length. Their purpose is tobreak up eddy currents that could be induced in metal plates 160 or 178from the gradient or RF fields of the MR scanner. By providing theseslots, heating due to such eddy currents is therefore minimized. It willbe obvious to those skilled in the art that these slots can be of anynumber, or of any shape. The use of said slots to minimize eddy currentsis applicable to any of the embodiments herein that embody metal plates.

FIG. 59 is a schematic diagram illustrating an in-line quadpolar DF-4connector 138A and that the same or two different values of capacitancecan be formed by the structure as illustrated in FIG. 52.

FIG. 60 shows a view of an in-line quadpolar IS-4 connector 138 whichincorporates both high voltage and low voltage connections.

FIG. 61 illustrates that the IS-4 connector of FIG. 60 (138) can alsoembody different configurations. In this case, four low voltageconnections. The IS-4 lead has significant advantages in that it offersa significant size reduction in an AIMD or abandoned lead cap 154 headerblock.

FIG. 62 is a cross-section drawing of the novel lead cap 154 of thepresent invention which is designed to terminate an abandoned IS-4 leadconnector 138. In this regard, it is very similar to the previousdescription for FIG. 46.

FIG. 63 is an isometric view of the IS-4 abandoned lead cap 154 of FIG.62.

FIG. 64 is a schematic diagram of the IS-4 abandoned lead cap 154 ofFIGS. 62 and 63.

FIG. 65 is an illustration of the abandoned lead cap of FIG. 63, showingthat any of the features of the present invention, including diverterand/or impeder elements 112 and 118, can be incorporated.

FIG. 66 is a schematic diagram of the IS-4 abandoned lead cap 154 ofFIG. 65.

FIG. 67 illustrates a paddle-shaped abandoned lead cap 154 embodying thepresent invention. It has a conductive energy dissipating surface 160shown on one side. A plurality of abandoned leads 104 is shown. Themethod of connection to the lead cap 154 can be by any other methodsdescribed herein. The leads can be directly connected (shorted) to theEDS surface 160. In the preferred embodiment, the leads 104 would beconnected through diverter elements 112 to the EDS surface.

FIG. 68 is a sectional view taken generally from section 68-68 from FIG.67. In this case, the diverter elements 112 are shown as capacitorelements 114. As described in previous drawings, all the lead wirescould be joined together and connected to a single capacitor element orthey could be individually routed to different capacitors asillustrated. By routing to different capacitors, the capacitance valuescan vary so that the capacitive reactance is carefully balanced to beopposite to the inductive reactance of the various implanted leads. Thisis in accordance with the maximum energy transfer principles of thepresent invention.

FIG. 69 is a drawing of a typical IS-1 lead 104, 132 that has beenunplugged from an AIMD, such as a cardiac pacemaker. Imagine that thepectoral pocket has been opened up by the surgeon, the pacemaker hasbeen removed and then the surgeon has taken a scissors or other surgicalcutting tool and cut off the end of the lead wire at location 70-70.Unfortunately, this snipping off of the lead connectors has been done inthe past. Hopefully, with the novel abandoned lead caps of the presentinvention, this practice can be discontinued in the future. However, forthese legacy abandoned leads, there is a need to be able to cap them ina way that improves their MRI safety.

FIG. 70 is the end view of the cut-off lead taken from section 70-70from FIG. 69. One can see that there are two coaxial or spiral woundlead wires 210 and 212 that run through the lead 104. The inside spirallead 212 is routed to the tip electrode 108. The outer spiral isconnected to the ring or anode electrode 110. The ring and tip electrodewires are electrically insulated from each other within the insulativebody of the lead 104. It would be very difficult for the surgeon toremove the insulation and separate out the ring and tip lead wires oncethe end has been cut off in this manner. Accordingly, a novel method isneeded to make contact with these embedded spiral wound or bifilar woundlead conductors.

FIG. 71 is a novel insert 214, shown inverted, with metal spikes 216 asshown. In a preferred embodiment, this would be made of injected moldedGrade-23 titanium. The material could also be of platinum, platinumiridium or any other suitable biocompatible material. In addition toinjection molding, the spike arrangement could be machined or formed.

The use of the structure, as shown in FIG. 71, is better understood byreferring to FIG. 72. The metal spike structure 214 (no longer inverted)is shown in cross-section taken generally from section 72-72 from FIG.71. The proximal end of the cut-off lead wire from FIG. 70 is firstinserted into the bore area 184 until it is fully seated at the end.Then the spike structure 214, which is slightly smaller in diameter thanthe threads, is inserted into the hole and pushed in through theinsulation of the lead 104. The various metal spikes 216 make electricalcontact in various places with both the spiral wound ring 210 and tip212 wiring inside of the lead 104. The set screw 174 is then put inplace, and using a torque tool, is firmly screwed in place which alsopresses against and seats the spikes 214. This has the affect ofshorting both the tip 212 and the ring 210 circuits together to metalplate 178. As previously described, a parasitic energy divertingcapacitance is formed between the metal plate 178 and the energydissipating surface 160. In an alternative embodiment, passive componentfrequency selective reactance components 118 and 112 can also beemployed in accordance with the present invention.

FIG. 73 illustrates another methodology of terminating abandoned leadsthat do not have a connector or who have had their connector removed.This is also applicable to various Neuromodulation lead systems that arehard wired (in other words, do not have a connector in the first place).One can see that there is a recess or counterbore 184 formed into theplastic non-conductive insulating surface 176 of the abandoned lead cap154. Once the lead 104 is inserted and the set screw 174′ is torqued,then a medical grade adhesive such as silicone 218 is used to fill inthe space around the cut-off lead illustrated in FIG. 70 around itscircumference into the angular space 184. This prevents moistureintrusion into the lead. A similar medical adhesive would be placed overthe set screw 174′ so that the lead end and the areas of electricalconnection to the tip 212 and ring 210 lead spirals are sealed off frombody fluid intrusion.

In a preferred embodiment, the pointed set screw structure 220 in FIG.73 would be sputtered or plated with a noble biocompatible surface tomake better electrical contact. Gold or platinum would be good choices.This is particularly important in the case where structure 174′,220 wasof titanium or similar metal that could form oxides on the surface.

FIG. 74 is very similar to FIG. 73 except that the set screw 174″ hasbeen specially modified to have a sharp pointed piercing tip 220 alongwith a secondary piercing ring 222. When inserted through an abandonedlead 104, the piercing tip 220 is designed to pierce through and bottomout as the set screw 174″ is being torqued. The distance between theprimary piercing tip 220 and the secondary piercing ring 222 isimportant such that the secondary piercing ring 222 pierces the ringelectrode 210. In this way, one can be assured that the set screw 174″is not screwed in too far and also that all of the imbedded leads 210and 212 are properly pierced so that a solid electrical connection ismade.

An additional feature illustrated in FIG. 74 is the use of silicon orequivalent O-rings 224. When the cutoff lead from FIG. 70 is insertedinto place, it is pressed firmly into these O-rings 224 such thatmoisture will not intrude. Illustrated in FIG. 74 is that any of theenergy diverter elements 112 or impeder elements 118 of the presentinvention can be incorporated. Also shown is an optional secondaryO-ring 226 to aid in mechanical capture and in moisture sealing of theinserted lead 104. Referring once again to FIG. 74, shown is a siliconcap 228 which is designed to be squeezed into place after the set screw174″ is seated. This prevents moisture intrusion from body fluid aroundthe thread surfaces of set screw 174′. This plug 228 would be set inplace with a small dab of medical grade adhesive.

FIG. 75 is an enlarged view of any of the energy dissipating surfaces160 of the novel abandoned lead caps 154 of the present invention. Shownis a convoluted surface such that the surface area of the energydissipating surfaces increases. This aids in both dissipation of RFenergy and in dissipation of thermal energy. Major advantages of theconvoluted surface as illustrated in FIG. 75 is that it will alsoincrease the MRI induced RF energy that is transferred and dissipatedfrom an implanted lead 104.

FIG. 76 is similar to FIG. 75 except that instead of convolutions, aroughened surface provides additional energy dissipation area. Theenergy dissipating surface 160 area has been roughened 230 to create ahigh surface area, through, for example, plasma etching, sputtering,chemical etching, or the like. A high surface area can also beaccomplished by porous coating deposits utilizing physical vapordeposition, chemical vapor deposition or electron beam depositionprocesses. Such porous coating deposits can include fractal coatings,metal nitrides, titanium nitrides, metal oxides, metal carbides, orvirtually anything that would provide a high surface or poroussubstrate. In addition, electrochemical deposition of porous coating,such as iridium-oxide, can also be utilized, as well as nucleate highsurface area morphologically structured coatings, such as whiskers,sub-micron filaments, tubes, nanotubes, or other morphologicalstructures such as columnar, titanium-nitride or iridium-oxide. Any ofthese types of surface conditionings can greatly increase the energydissipating surface area.

FIG. 77, which is similar to FIG. 76, illustrates the use of carbonnanotubes or fractal coatings 232 to increase the surface area andtherefore the energy dissipation.

In summary, the present invention involves terminating abandoned AIMDleads utilizing novel abandoned lead caps associated with an energydissipating surface. In general, the abandoned lead cap includes anelectrically conductive housing/electrode which works in combinationwith frequency selective circuits so that the housing of the abandonedlead cap works as an energy dissipating surface. The energy dissipatingsurface may be disposed within the blood flow of a patient or comprise aplurality of spaced-apart energy dissipating surfaces. The energydissipating surface may also include one or more slots for reducing eddycurrent heating therein. Many types of energy dissipating surfaces maybe utilized such as convolutions, roughened surfaces and carbonnanotubes. The abandoned lead cap of the present invention may alsoinclude an RFID tag associated therewith.

The system for terminating an abandoned implanted lead to minimizeheating in a high power electromagnetic field environment in accordancewith the present invention, comprises: (1) an implanted abandoned leadhaving a proximal end and a distal end, and impedance characteristics ata selected RF frequency or RF frequency band; (2) an abandoned lead caphaving an energy dissipating surface (EDS surface) which is associatedwith the proximal end of the implanted abandoned lead; and (3) an energydiversion circuit conductively coupling the implanted abandoned lead tothe energy dissipating surface to facilitate transfer to the energydissipating surface of high frequency energy induced on the implantedabandoned lead at the selected RF frequency or frequency band.

The impedance of the abandoned lead cap may be balanced to the implantedlead impedance such that maximum energy is dissipated within theabandoned lead cap itself. In this case, thermal energy can bedissipated inside the abandoned lead cap, which has a controlled thermalmass and a controlled rate of temperature rise.

The present invention also includes methods of attachment to anabandoned lead that has been cut off as well as to an abandoned leadproximal connector.

Although several embodiments of the invention have been described indetail for purposes of illustration, various modifications may be madewithout departing from the scope and spirit of the invention.Accordingly, the invention is not to be limited, except as by theappended claims.

1. A lead cap for terminating an abandoned implanted lead to therebyminimize heating of the lead in a high power electromagnetic fieldenvironment, the lead cap comprising: a) a housing providing an energydissipating surface that is connectable to an implanted abandoned lead;and b) an energy diversion circuit electrically connected to the energydissipating surface, wherein when the housing is connected to anabandoned lead, the energy diversion circuit facilitates transfer ofhigh frequency energy induced on the lead at an RF frequency orfrequency band to the energy dissipating surface, and c) wherein theenergy diversion circuit has at least one of a first reactance and afirst impedance that is at least partially vectorally opposite to asecond reactance and a second impedance of the lead.
 2. The lead cap ofclaim 1, wherein the energy diversion circuit comprises one or morepassive electronic network components whose first impedance are at leastpartially tuned to the second impedance of an implanted abandoned lead.3. The lead cap of claim 2, wherein the RF frequency or frequency bandcomprises an MRI frequency or MRI frequency band.
 4. The lead cap ofclaim 1 wherein the energy diversion circuit has a capacitive reactancethat is generally equal to and opposite an inductive reactance of animplanted lead.
 5. The lead cap of claim 1 wherein the energy diversioncircuit comprises at least one series resonant LC trap filter.
 6. Thelead cap of claim 1 wherein the energy dissipating surface comprisesconvolutions or fins provided on the housing for increasing the surfacearea thereof.
 7. The lead cap of claim 1 wherein the energy dissipatingsurface comprises a plurality of spaced-apart energy dissipatingsurfaces.
 8. The lead cap of claim 1 wherein the energy diversioncircuit includes at least one non-linear circuit element.
 9. The leadcap of claim 1 wherein the energy diversion circuit comprises a highpass filter.
 10. The lead cap of claim 9, wherein the high pass filteris adapted to prevent low frequency gradient field-induced energy in animplanted lead from passing through the energy diversion circuit to theenergy dissipating surface.
 11. The lead cap of claim 10 wherein the lowfrequency gradient field-induced energy is below 4 kHz.
 12. The lead capof claim 9, wherein the high pass filter is selected from the groupconsisting of a capacitor, a resistor in series with a capacitor, and anL-C trap filter.
 13. The lead cap of claim 1, wherein the energydiversion circuit comprises a direct connection or short to the energydissipating surface.
 14. The lead cap of claim 1, including an RFID tagmounted on or housed within the housing.
 15. The lead cap of claim 1,wherein the housing providing the energy dissipating surface comprisesgrade 5 or grade 23 titanium.
 16. The lead cap of claim 1, wherein thehousing providing the energy dissipating surface includes one or moreslots for reducing eddy current heating therein.
 17. The lead cap ofclaim 1, wherein the housing is connectable to an end of at least oneimplanted abandoned lead comprising an IS standard connector portselected from the group consisting of IS-1, IS4, DF4, and DF-1.
 18. Thelead cap of claim 1, wherein the energy diversion circuit, comprises atleast one discrete electronic network component selected from the groupconsisting of an inductor, a capacitor, a resistor, a diode, and a pindiode.
 19. The lead cap of claim 1, wherein the housing includes a sealthat is connectable to a lead to isolate a connected end of the leadfrom body fluid.
 20. The lead cap of claim 1, wherein the housingproviding the energy dissipating surface comprises a material capable ofbeing visualized during a magnetic resonance scan.
 21. The lead cap ofclaim 1 wherein the energy diversion circuit comprises a low pass orhigh pass filter.
 22. The lead cap of claim 1 wherein the energydiversion circuit comprises a bandstop or bandpass filter.
 23. The leadcap of claim 1 being positionable within a blood flow.
 24. The lead capof claim 14 wherein the RFID tag includes retrievable informationselected from the group consisting of a lead cap manufacture and modelnumber, a lead manufacture and model number, and MRI compatibility oflead and the lead cap.
 25. A lead cap for terminating an abandonedimplanted lead to thereby minimize heating of the lead in a high powerelectromagnetic field environment, the lead cap comprising: a) an energydissipating substrate; and b) an energy diversion circuit electricallyconnected to the energy dissipating substrate, wherein the energydiversion circuit is conductively connectable to an implanted abandonedlead to thereby facilitate transfer of high frequency energy induced onthe lead at an RF frequency or frequency band to the energy dissipatingsubstrate, and d) wherein the energy diversion circuit has at least oneof a first reactance and a first impedance that is at least partiallyvectorally opposite to a second reactance and a second impedance of thelead.
 26. The lead cap of claim 25 wherein the energy diversion circuitcomprises one or more passive electronic network components whose firstimpedance are at least partially tuned to the second impedance of animplanted abandoned lead.
 27. The lead cap of claim 26 wherein the RFfrequency or frequency band comprises an MRI frequency or an MRIfrequency band.
 28. The lead cap of claim 25 wherein the energydiversion circuit has a capacitive reactance that is generally equal toand opposite an inductive reactance of an implanted lead.
 29. The leadcap of claim 25 wherein the energy diversion circuit comprises at leastone series resonant LC trap filter.
 30. The lead cap of claim 25 whereinthe energy diversion circuit includes at least one non-linear circuitelement.
 31. The lead cap of claim 25 wherein the energy diversioncircuit comprises a high pass filter.
 32. The lead cap of claim 31wherein the high pass filter is adapted to prevent low frequencygradient field-induced energy in an implanted lead from passing throughthe energy diversion circuit to the energy dissipating substrate. 33.The lead cap of claim 31 wherein the high pass filter is selected fromthe group consisting of a capacitor, a resistor in series with ancapacitor, and an L-C trap filter.
 34. The lead cap of claim 25 whereinthe energy diversion circuit comprises a direct connection or short tothe energy dissipating substrate.
 35. The lead cap of claim 25 includingan RFID tag mounted on or housed within the energy dissipatingsubstrate.
 36. The lead cap of claim 25 wherein the energy dissipatingsubstrate comprises grade 5 or grade 23 titanium.
 37. The lead cap ofclaim 25 wherein the energy diversion circuit comprises at least onediscrete electronic network component selected from the group of aninductor, a capacitor, a resistor, a diode, and a pin diode.
 38. Thelead cap of claim 25 wherein the energy dissipating substrate comprisesa material capable of being visualized during a magnetic resonance scan.39. The lead cap of claim 25 wherein the energy diversion circuitcomprises a low pass or high pass filter.
 40. The lead cap of claim 25wherein the energy diversion circuit comprises a bandstop or bandpassfilter.
 41. A lead cap for terminating an abandoned implanted lead tothereby minimize heating of the lead in a high power electromagneticfield environment, the lead cap comprising: a) an energy dissipatingsurface that is connectable to an implanted abandoned lead; and b) anenergy diversion circuit electrically connected to the energydissipating surface, wherein when so connected, the energy diversioncircuit, facilitates transfer of high frequency energy induced on thelead at an RF frequency or frequency band to the energy dissipatingsurface, and c) wherein the energy diversion circuit has a capacitivereactance that is generally equal to and opposite an inductive reactanceof the lead.